changed default hover+selection mode as follows:

- by default, mouse hover+selection is ON for the geometry, but only
  mouse selection is ON for the mesh

- with Shift+Escape, one can activate "full" selection mode, i.e., also
  allow mouse hover for the mesh

I've disabled the hover over mesh stuff due to slow performance on both
of the Windows machines I have access to over here.

With these new settings I get interactive performance (about 4 frames
per second) when visualization a 6 million tetrahedra mesh, with all the
edges inside visible, lights turned on, etc. This takes about 600Mb of
RAM.