Make engine loadable during session

Make engine loadable during session

A "Load Engine ..." menu item is added to the Engine menu, which pops
up a dialog where one can replace the first or second engine by a new
one, without destroying the game state. Switching from -ncp to engine
mode needed activation of some menu items in SetGNUMode, and
re-enabling clockMode. It was also important that noChessProgram is
cleared before re-initializing the ChessProgramState.
  The Load-Engine dialog can select from the list of installed
engines, and also add a newly specified engine to this list. In this
case it saves the directory, hasBook, and isUCI info with the engine,
and optionally the current variant. When loading an engine with a
specified variant, it resets the game and switches to the new variant.