now, others can be added later (sticky, shape keys). Beside one small fix
for knife exact vertex group interpolation, is intended to work the same
as before.
Also fixes bug #5200, related to editmode undo and vertex groups. And
corrects the editmode to faceselect mode selection conversion, that was
broken in a previous commit.
the CustomData module from the modifier stack rewrite, but with additions
to make it also usable in edit mode. Some of the datatypes from that
module were move to a DNA header file, they are not saved to file now, but
will be soon.
The only code that wasn't abstracted is the uv collapse / merging code. It
is rather complicated, will look into that in the future.
There should be no user level changes.
