Centralise palette setup into terminal.c.

Now terminal.c makes nearly all the decisions about what the colour
palette should actually contain: it does the job of reading the
GUI-configurable colours out of Conf, and also the job of making up
the rest of the xterm-256 palette. The only exception is that TermWin
can provide a method to override some of the default colours, which on
Windows is used to implement the 'Use system colours' config option.

This saves code overall, partly because the front ends don't have to
be able to send palette data back to the Terminal any more (the
Terminal keeps the master copy and can answer palette-query escape
sequences from its own knowledge), and also because now there's only
one copy of the xterm-256 palette setup code (previously gtkwin.c and
window.c each had their own version of it).

In this rewrite, I've also introduced a multi-layered storage system
for the palette data in Terminal. One layer contains the palette
information derived from Conf; the next contains platform overrides
(currently just Windows's 'Use system colours'); the last one contains
overrides set by escape sequences in the middle of the session. The
topmost two layers can each _conditionally_ override the ones below.
As a result, if a server-side application manually resets (say) the
default fg and bg colours in mid-session to something that works well
in a particular application, those changes won't be wiped out by a
change in the Windows system colours or the Conf, which they would
have been before. Instead, changes in Conf or the system colours alter
the lower layers of the structure, but then when palette_rebuild is
called, the upper layer continues to override them, until a palette
reset (ESC]R) or terminal reset (e.g. ESC c) removes those upper-layer
changes. This seems like a more consistent strategy, in that the same
set of configuration settings will produce the same end result
regardless of what order they were applied in.

The palette-related methods in TermWin have had a total rework.
palette_get and palette_reset are both gone; palette_set can now set a
contiguous range of colours in one go; and the new
palette_get_overrides replaces window.c's old systopalette().

putty.h

@@ -63,9 +63,9 @@
  * Since the OSC 4 encoding contains the full set of colours used in
  * the terminal display, that's the encoding used by front ends to
  * store any actual data associated with their palette entries. So the
- * TermWin palette_{set,get} methods use that encoding, and so does
- * the bitwise encoding of attribute words used in terminal redraw
- * operations.
+ * TermWin palette_set and palette_get_overrides methods use that
+ * encoding, and so does the bitwise encoding of attribute words used
+ * in terminal redraw operations.
  *
  * The Conf encoding, of course, is used by config.c and settings.c.
  *
@@ -1204,6 +1204,10 @@ bool console_set_trust_status(Seat *seat, bool trusted);
 int filexfer_get_userpass_input(Seat *seat, prompts_t *p, bufchain *input);
 bool cmdline_seat_verbose(Seat *seat);
 
+typedef struct rgb {
+    uint8_t r, g, b;
+} rgb;
+
 /*
  * Data type 'TermWin', which is a vtable encapsulating all the
  * functionality that Terminal expects from its containing terminal
@@ -1268,10 +1272,16 @@ struct TermWinVtable {
     void (*move)(TermWin *, int x, int y);
     void (*set_zorder)(TermWin *, bool top);
 
-    /* Palette-handling functions. Palette indices are in OSC 4 encoding. */
-    bool (*palette_get)(TermWin *, unsigned n, int *r, int *g, int *b);
-    void (*palette_set)(TermWin *, unsigned n, int r, int g, int b);
-    void (*palette_reset)(TermWin *);
+    /* Set the colour palette that the TermWin will use to display
+     * text. One call to this function sets 'ncolours' consecutive
+     * colours in the OSC 4 sequence, starting at 'start'. */
+    void (*palette_set)(TermWin *, unsigned start, unsigned ncolours,
+                        const rgb *colours);
+
+    /* Query the front end for any OS-local overrides to the default
+     * colours stored in Conf. The front end should set any it cares
+     * about by calling term_palette_override. */
+    void (*palette_get_overrides)(TermWin *);
 
     void (*get_pos)(TermWin *, int *x, int *y);
     void (*get_pixels)(TermWin *, int *x, int *y);
@@ -1325,12 +1335,11 @@ static inline void win_move(TermWin *win, int x, int y)
 { win->vt->move(win, x, y); }
 static inline void win_set_zorder(TermWin *win, bool top)
 { win->vt->set_zorder(win, top); }
-static inline bool win_palette_get(TermWin *win, unsigned n,
- int *r, int *g, int *b) { return win->vt->palette_get(win, n, r, g, b); }
-static inline void win_palette_set(TermWin *win, unsigned n,
- int r, int g, int b) { win->vt->palette_set(win, n, r, g, b); }
-static inline void win_palette_reset(TermWin *win)
-{ win->vt->palette_reset(win); }
+static inline void win_palette_set(
+    TermWin *win, unsigned start, unsigned ncolours, const rgb *colours)
+{ win->vt->palette_set(win, start, ncolours, colours); }
+static inline void win_palette_get_overrides(TermWin *win)
+{ win->vt->palette_get_overrides(win); }
 static inline void win_get_pos(TermWin *win, int *x, int *y)
 { win->vt->get_pos(win, x, y); }
 static inline void win_get_pixels(TermWin *win, int *x, int *y)
@@ -1763,6 +1772,8 @@ void term_keyinputw(Terminal *, const wchar_t * widebuf, int len);
 void term_get_cursor_position(Terminal *term, int *x, int *y);
 void term_setup_window_titles(Terminal *term, const char *title_hostname);
 void term_notify_minimised(Terminal *term, bool minimised);
+void term_notify_palette_overrides_changed(Terminal *term);
+void term_palette_override(Terminal *term, unsigned osc4_index, rgb rgb);
 
 typedef enum SmallKeypadKey {
     SKK_HOME, SKK_END, SKK_INSERT, SKK_DELETE, SKK_PGUP, SKK_PGDN,