Blender Git Loki

Keymap: make plane track corners draggable with lmb

Previously when clicking and dragging with LMB you would only move the entire
plane track. In order to move the corners independently you would have to use
your right mouse button. This would also prevent the context menu to show up.

Now LMB click and drag on corners moves them. If you LMB click and drag one of
the 4 edges of the plane track you would move the entire plane track.

Differential Revision: https://developer.blender.org/D5519