Gdk::Color: Deprecate this.

* gdk/src/cellrenderertext.hg: Deprecate the properties that use
  Gdk::Color now that they are deprecated (and replaced) in the C API.
* gdk/src/color.hg: Deprecate this now that it is only used by us for
  deprecated API. The C API still uses GdkColor, but we can already
  deprecate it.
Device: Document some methods.
Gdk::Pixbuf: Document some methods.