Add context aware fill operator

[jlampel]

RetopoFlow should have a one-click, context aware way to fill selected areas with quads. The full details still need to be worked out, but the operator should be able to grid fill, bridge, create face, new edge from verts (previously F2), or regular fill depending on the selection and then snap the result to the surface using Face Nearest snapping.

User request:

• When nothing is selected and four vertices are nearby, fill a quad. Check out quad draw for inspiration