Add new PDB group "drawable_edit" which has all procedures from the
"edit" group which are not cut/copy/paste.
The new group's procedures don't have opacity, paint_mode
etc. arguments but take them from the context instead. Unlike the old
gimp-edit-fill, gimp-drawable-edit-fill now uses the context's opacity
and paint_mode.
The new gimp-drawable-edit-gradient-fill procedure uses even more
context properties which are also newly added with this commit
(gradient_color_space, gradient_repeat_mode, gradient_reverse).
And some cleanup in context.pdb.
This is still WIP, nothing in the edit group is depcreated yet.
