Blender Git Loki

UI: Pan 2D Region When Dragging to Boundary

This adds a modal operator called Edge Pan, which is meant to run invisibly
while something inside a region is being dragged. This patch applies this
to dragging panels, but it can be used elsewhere too.

The speed (which is defined relative to how far the mouse is beyond the
boundary) and delay are easily adjustible. and the speed also increases
smoothly from a start value to a max to make it feel more interactive
and less robotic.

Differential Revision: https://developer.blender.org/D7465