Introduce Emacs_Color struct and typedef

Introduce Emacs_Color struct and typedef

This avoids clashing with the XColor struct from X.

* src/dispextern [HAVE_X_WINDOWS]: Define Emacs_Color alias.
[!HAVE_X_WINDOWS]: Rename XColor compatibility struct to
Emacs_Color. Remove unused fields.

* src/gtkutil.c:
* src/gtkutil.h:
* src/image.c:
* src/nsterm.h:
* src/nsterm.m:
* src/termhooks.h:
* src/w32fns.c:
* src/w32term.c:
* src/w32term.h:
* src/xfaces.c:
* src/xfns.c:
* src/xterm.h: Use Emacs_Color over XColor outside of X-specific
sections.