quad_edge_mesh: Moved is_updated variable from blender_quad_edge_mesh.BlenderQEVertex to quad_edge_mesh.QEVertex

Hard-crash bug-workaround
blender_quad_edge_mesh: calling calc_tessface() TWICE sequentially seems to workaround the hard-crash I was experiencing.

Created two buttons on the panel to execute/cleanup BlendSeg. These might not make it into final but it's good to know they seem to be working.