Blender Git Loki

Bread-first bmesh walkers:
- walkers can be run in breadth-first order or depth-first order (previously there was only depth-first)
- walkers keep track of current depth (with this + breadth-first, select nth implementation is now trivial)
- new connected vertex walker (similar to shell walker, but returns vertices instead of edges)
- shell walker can be started from a vertex (in which case the walker starts by queueing all the edges incident on the vertex) or from an edge (walker starts by queueing the single edge)
- bug fix for walker reset (was emptying states, but not clearing the visithash)
- bug fix for select nth (indexing for deselection was walking through *all* connected elmeents, to match trunk it should only walk through *selected* connected elements)