app,libgimp,pdb: add a "drawable_color" PDB group

Which contains all the API from the "color" group, but with a
gimp_drawable namespace and with support for high bit depths. The
group is actually a copy of "color" with cruft removed and some API
ported to using float instead of integer API.

Deprecated all "color" functions that already have a ported version in
"drawable_color".

This is unfinished WIP, some functions in "drawable_color" still have
the old API.