Introduce Emacs_GC struct and typedef

Introduce Emacs_GC struct and typedef

* src/dispextern.h [HAVE_X_WINDOWS]: Alias Emacs_GC to XGCValues.
[!HAVE_X_WINDOWS]: Define Emacs_GC, GCForeground, and GCBackground.

* src/nsgui.h:
* src/w32gui.h:Remove obsolete XGCValues, GC, GCForeground,
GCBackground, and GCFont definitions.

* src/w32fns.c (w32_make_gc): Do not set unused font field.

* src/w32term.c: Use Emacs_GC over XGCValues. Do not set unused font
field.

* src/xfaces.c: Use Emacs_GC over XGCValues and GC.