Blender Git Loki

Edit Mesh: Poly build tool improvements

This commit changes the functionality of the Poly build tool to make it more suitable for retopology tasks:
- Click and drag from a boundary edge extrudes a new quad
- Click and drag on vertices tweaks the position
- Ctrl + click adds geometry. There is a geometry preview in the gizmo. It also can automatically convert triangles to quads.
- Shift + click deletes mesh elements (faces or vertices)
- Updated preselection code. Different mesh elements take priority depending on the selected action.

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D5573