Move all window-title management into Terminal.

Previously, window title management happened in a bipartisan sort of
way: front ends would choose their initial window title once they knew
what host name they were connecting to, but then Terminal would
override that later if the server set the window title by escape
sequences.

Now it's all done the same way round: the Terminal object is always
where titles are invented, and they only propagate in one direction,
from the Terminal to the TermWin.

This allows us to avoid duplicating in multiple front ends the logic
for what the initial window title should be. The frontend just has to
make one initial call to term_setup_window_titles, to tell the
terminal what hostname should go in the default title (if the Conf
doesn't override even that). Thereafter, all it has to do is respond
to the TermWin title-setting methods.

Similarly, the logic that handles window-title changes as a result of
the Change Settings dialog is also centralised into terminal.c. This
involved introducing an extra term_pre_reconfig() call that each
frontend can call to modify the Conf that will be used for the GUI
configurer; that's where the code now lives that copies the current
window title into there. (This also means that GTK PuTTY now behaves
consistently with Windows PuTTY on that point; GTK's previous
behaviour was less well thought out.)

It also means there's no longer any need for Terminal to talk to the
front end when a remote query wants to _find out_ the window title:
the Terminal knows the answer already. So TermWin's get_title method
can go.

terminal.c

@@ -1515,6 +1515,17 @@ void term_copy_stuff_from_conf(Terminal *term)
     }
 }
 
+void term_pre_reconfig(Terminal *term, Conf *conf)
+{
+
+    /*
+     * Copy the current window title into the stored previous
+     * configuration, so that doing nothing to the window title field
+     * in the config box doesn't reset the title to its startup state.
+     */
+    conf_set_str(conf, CONF_wintitle, term->window_title);
+}
+
 /*
  * When the user reconfigures us, we need to check the forbidden-
  * alternate-screen config option, disable raw mouse mode if the
@@ -1565,6 +1576,16 @@ void term_reconfig(Terminal *term, Conf *conf)
         }
     }
 
+    {
+        const char *old_title = conf_get_str(term->conf, CONF_wintitle);
+        const char *new_title = conf_get_str(conf, CONF_wintitle);
+        if (strcmp(old_title, new_title)) {
+            sfree(term->window_title);
+            term->window_title = dupstr(new_title);
+            win_set_title(term->win, term->window_title);
+        }
+    }
+
     conf_free(term->conf);
     term->conf = conf_copy(conf);
 
@@ -1658,6 +1679,25 @@ void term_clrsb(Terminal *term)
 
 const optionalrgb optionalrgb_none = {0, 0, 0, 0};
 
+void term_setup_window_titles(Terminal *term, const char *title_hostname)
+{
+    const char *conf_title = conf_get_str(term->conf, CONF_wintitle);
+    sfree(term->window_title);
+    sfree(term->icon_title);
+    if (*conf_title) {
+        term->window_title = dupstr(conf_title);
+        term->icon_title = dupstr(conf_title);
+    } else {
+        if (title_hostname)
+            term->window_title = dupcat(title_hostname, " - ", appname);
+        else
+            term->window_title = dupstr(appname);
+        term->icon_title = dupstr(term->window_title);
+    }
+    win_set_title(term->win, term->window_title);
+    win_set_icon_title(term->win, term->icon_title);
+}
+
 /*
  * Initialise the terminal.
  */
@@ -1750,6 +1790,9 @@ Terminal *term_init(Conf *myconf, struct unicode_data *ucsdata, TermWin *win)
 
     term->bracketed_paste_active = false;
 
+    term->window_title = dupstr("");
+    term->icon_title = dupstr("");
+
     return term;
 }
 
@@ -1804,6 +1847,9 @@ void term_free(Terminal *term)
 
     conf_free(term->conf);
 
+    sfree(term->window_title);
+    sfree(term->icon_title);
+
     sfree(term);
 }
 
@@ -2794,15 +2840,21 @@ static void do_osc(Terminal *term)
         switch (term->esc_args[0]) {
           case 0:
           case 1:
-            if (!term->no_remote_wintitle)
+            if (!term->no_remote_wintitle) {
                 win_set_icon_title(term->win, term->osc_string);
+                sfree(term->icon_title);
+                term->icon_title = dupstr(term->osc_string);
+            }
             if (term->esc_args[0] == 1)
                 break;
             /* fall through: parameter 0 means set both */
           case 2:
           case 21:
-            if (!term->no_remote_wintitle)
+            if (!term->no_remote_wintitle) {
                 win_set_title(term->win, term->osc_string);
+                sfree(term->window_title);
+                term->window_title = dupstr(term->osc_string);
+            }
             break;
           case 4:
             if (term->ldisc && !strcmp(term->osc_string, "?")) {
@@ -4422,7 +4474,7 @@ static void term_out(Terminal *term)
                                 if (term->ldisc &&
                                     term->remote_qtitle_action != TITLE_NONE) {
                                     if(term->remote_qtitle_action == TITLE_REAL)
-                                        p = win_get_title(term->win, true);
+                                        p = term->icon_title;
                                     else
                                         p = EMPTY_WINDOW_TITLE;
                                     len = strlen(p);
@@ -4437,7 +4489,7 @@ static void term_out(Terminal *term)
                                 if (term->ldisc &&
                                     term->remote_qtitle_action != TITLE_NONE) {
                                     if(term->remote_qtitle_action == TITLE_REAL)
-                                        p = win_get_title(term->win, false);
+                                        p = term->window_title;
                                     else
                                         p = EMPTY_WINDOW_TITLE;
                                     len = strlen(p);