Added custom face data support in edit mode. The code used to do this is

Added custom face data support in edit mode. The code used to do this is
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.