Task: https://developer.blender.org/T37960
Thanks a lot :)
Those only cover the current set of brushes, soc-paint brushes will be
commited on that branch
Most of the tool code was already in place, only significant
functionality change is that anchored stroke works now.
TODO:
* Gave it a new icon, but could use a better one
* Default .blend should have a clay strips brush
* Turned off 'Front-Face Only' for all brushes, it needs more testing
* Added brush icons for other paint modes
* Moved 'tool' panel to bottom of all paint modes
* Moved 'appearance' panel to be next to bottom
* Moved brush selector panel to top of all modes
* Closed all panels except the brush selector panel
* Turned off X symmetry
