[Bf-committers] BMesh: implement select nth, separate loose parts, and a few bug fixes

[Andrew Wiggin]

Attached is a bmesh patch that 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)

For the extrude faces flash, the fix may not be in the right place, so any
advice on where to move this, or possibly update flags that would be handled
somewhere higher up the chain by automatically recalculating
normals/tesselations would be great.

For select nth, I needed some int tmps attached to the elements. I pushed a
flag layer and used the index of the layer. Probably this is the wrong way
to do this, would like to hear advice on the right way and I could fix this.

This is my first ever patch and I know I'm not going to get everything right
right away, so please don't hesitate criticize any aspects (esp. code style,
comment style)