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().

terminal.h

@@ -345,6 +345,21 @@ struct terminal_tag {
 
     char *window_title, *icon_title;
     bool minimised;
+
+    /* Multi-layered colour palette. The colours from Conf (plus the
+     * default xterm-256 ones that don't have Conf ids at all) have
+     * lowest priority, followed by platform overrides if any,
+     * followed by escape-sequence overrides during the session. */
+    struct term_subpalette {
+        rgb values[OSC4_NCOLOURS];
+        bool present[OSC4_NCOLOURS];
+    } subpalettes[3];
+#define SUBPAL_CONF 0
+#define SUBPAL_PLATFORM 1
+#define SUBPAL_SESSION 2
+
+    /* The composite palette that we make out of the above */
+    rgb palette[OSC4_NCOLOURS];
 };
 
 static inline bool in_utf(Terminal *term)