These functions are implementations of the AtkText api on top
of a PangoLayout, and are intended to replace GailTextUtil.
Since gtkpango.h is a private header, also remove the individual
inclusion prohibition.
This includes the addition of a "small" helper function,
_gtk_pango_fill_layout() that ignores color information. This
functionality is not available inside Pango and until that happens, we
need this fix. The bug is filed at:
https://bugzilla.gnome.org/show_bug.cgi?id=624917
