Blender Git Loki

Changes to Stencil control:

* Add a poll function that only activates the operator when a stencil
brush is active
* Change shortcuts to RMouse - translate, Shift - RMouse Scale, Ctrl -
RMouse, rotate. MUCH faster and simpler workflow.
* Change shortcut of colour sampling to S. Sampling is important but not
as important as warranting the Right Mouse button.
* Add a notifier so that toolbar gets updated texture rotation when
rotating the stencil

Also,
* Slight rearrangement of options so that jittering is more accessible
(easily more often used than smooth stroke)