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.
GSOC 2017 by Darshan Kadu, see: D2859.
This is a partial merge of some of the features from
the soc-2017-vertex_paint branch.
- Alpha painting & drawing.
- 10 new color blending modes.
- Support for vertex select in vertex paint mode.