Fix changing colours in Change Settings.

Since ca9cd983, changing colour config mid-session had no effect
(until the palette was reset for some other reason). Now it does take
effect immediately (provided that the palette has not been overridden by
escape sequence -- this is new with ca9cd983).

This changes the semantics of palette_reset(): the only important
parameter when doing that is whether we keep escape sequence overrides
-- there's no harm in re-fetching config and platform colours whether or
not they've changed -- so that's what the parameter becomes (with a
sense that doesn't require changing the call sites). The other part of
this change is actually remembering to trigger this when the
configuration is changed.

(cherry picked from commit 1e726c94)