Added option in shift-g to select verts by number of connected edges (valence).

Fixed crash in iter api, triggered by calling index_update() on the loop py seq in the py api. Note, I didn't fix the underlying problem (bad iter type) so loops.index_update() still isn't working (it's just not crashing).

Fix related to #30859: bmesh face splitting would set the first loop of the new
faces to a loop quite different than the original first loop. This makes e.g.
duplifaces give rotated results after subdivide or loopcut.

Now it tries to find a first loop that is similar to the old one, to try to keep
the rotations the same.

Changed BM_mesh_bm_from_me to copy customdata layout for empty meshes.

bmesh: fixed crash in bevel

bmesh: fixed merge issues with navmesh (though I've not tested if it works yet).
also fixed a small crasher in bridge.

=bmesh= fixed uv unwrap tool

=bmesh= sculpt without multires updates properly now. not sure I did this correctly, though.

=bmesh= fixed loop select for wire edges

=bmesh= knife tool applies to modifier cage now, also fixed some bugs in it