gdk-color-intern colormap

[Kevin Ryde]

By way of warning, I propose to make gdk-color-intern just call
gtk_widget_get_default_colormap.

I see the intention was to get gtk_widget_peek_colormap, but that
function has never been made public, so guile-gtk has always just been
falling back on gtk_widget_get_default_colormap.

On that basis this is a non-change, just settling on the current
situation.

There seems no way to access the colormap stack to do a peek_colormap
by other means.  No great loss really, anyone mucking about with
multiple colormaps should be smart enough to allocate in the right
one.