Automatically play moves of clicked PV in analyze mode

Automatically play moves of clicked PV in analyze mode

The Shift key now no longer needs to be pressed to play moves from the
PV in analyze mode (so it could be given back its original meaning of
starting a variation with the entered moves). In analyze mode the PV
walk begins after the first move, so a static right-click plays the
first PV move. To just peek the PV you have to walk to the start of it
before releasing the mouse button. In other modes the PV walk
stillstarts at the very end.