Color: Fix the build with --enable-warnings=fatal.

* gdk/src/color.hg: Disable GTK+ deprecation warnings for the .cc file
  so we can still build our (also deprecated) implementation.
Deprecate Alignment because it is deprecated in GTK+.

* gtk/src/alignment.hg: Deprecate this.
* demos/gtkmm-demo/example_colorsel.c:
* demos/gtkmm-demo/example_icontheme.cc:
* demos/gtkmm-demo/example_images.cc:
* demos/gtkmm-demo/example_textview.cc: Replace use of
  Gtk::Alignment with Widget h/valign.
3.13.2