Notes:
- Changes in paint_vertex.c were simple to merge, mainly related on passing
evaluation context.
- Conflicts in EditDM and drawmesh.c are solved using code from blender2.8
branch. Those areas are deprecated and not to be used in final release.
However, it's possible that some reference code from master is lost, so
keep attention when adding alpha support for vertex painting.
paint_vertex.c was getting too big, move all code unrelated to
mode switching and modal painting into their own files.
Also replace vertex-color operators region redraw tag /w notifiers.
