Some more Bmesh custom data functions and
structures. This still does not do anything
yet because the various conversion functions
don't bother making use of the new custom data
functions. Hooking them up should be fairly
simple though.
Also note that the custom data code is mostly
copy/pasted from the existing custom data
functions for editmode with a few modifications.
Duplicating code like this isn't nice, but I
felt it was better to keep things for Bmesh
'standalone' for the moment and take only what is
immediatly needed instead of creating
a tangle of interdependant code.
