Blender Git Loki

UI: Add Sculpt Session info to stats

This adds the vertex and face count info to the scene stats in sculpt
mode. These stats count the active vertices and faces in the
sculptsession for the active object. This has the following advantages:
- It is possible to know how many vertices the sculptsession has active
comparted to the vertex count of the entire scene from sculpt mode
- When sculpting with constructive modifiers, these stats will report the
number of vertices that you can actually sculpt with, instead of the
vertex count of the modified mesh and the entire scene.

Reviewed By: sergey, dbystedt

Differential Revision: https://developer.blender.org/D9623