added BoardView::setBestMove()
more work on bestmove.
BoardView checks for spacebar to execute, once it has been selected. Not clear, however, how to delegate the spacebar from the MainWindow, without affecting it's other uses.
handled the global space bar in mainwindow -> BoardView::execBestMove
took out global spacebar: interferes with other widgets.
TODO: redesign all the event handling. (Why are there so many filters?)