Fix PV walking in analysis mode

Fix PV walking in analysis mode

ParsePV assumed that the PV would always have been produced by an engin
thinking about the final position of the game (or the one before it). In
analysismode, however, the engine can be analysing positions internal to
the game, and the PV would be considered invalid. This is fixed by
cutting of the tail off the game (shelving it in variation fashion)
before starting the PV walk, and reverting afterwards.
  For this the PushTail and PopTail routines had to be split into an
'inner' part, doing the pushing and popping, and an outer part, handling
menu graying and popdowns (which is undesirable in in this application).