Fix some bad interactions with Unity wm.

M-mouse-6 seems to trigger lowering of the active window (effectively
hiding emacs when I try to use this), and C-mouse-6 seems to do
nothing (Unity eats it and doesn't let Emacs respond to it), so I'm
now settling on the Shift key as the modifier for this. It probably
would make more sense to set the mark with the shift key but I'd
rather leave it as is because I move point more often than I set the
mark.