Make some static text in GTK dialogs selectable.

I've made the licence text, the About box, and the host key dialog
into GTK selectable edit controls. (The former because it contains a
lot of text; the About box because pasting version numbers into bug
reports is obviously useful; the host key because of the fingerprint.)

unix/gtkwin.c

@@ -4345,7 +4345,7 @@ void change_settings_menuitem(GtkMenuItem *item, gpointer data)
                               errmsg);
                 messagebox(inst->window, "Font setup error", msgboxtext,
                            string_width("Could not change fonts in terminal window:"),
-                           "OK", 'o', +1, 1,
+                           FALSE, "OK", 'o', +1, 1,
                            NULL);
                 sfree(msgboxtext);
                 sfree(errmsg);