Blender Git Loki

implement select nth, separate loose parts, and a few bug fixes

This code is from Andrew Wiggen (ender79) and reviewed by me.

His comments:

Implements some tools that were marked TODO:
- select nth
- separate loose parts

And also fixes a few bugs;
- extrude and move on normals causes faces to disappear until the move starts
- hiding verts/edges/faces does not deselect them
- deleting a selection sometimes deletes too much (e.g. a solid cube and a wire cube build of only edges, join them on a single edge, select only the faces of the solid cube and delete, some of the deselected edges from the wire cube were also being deleted)