Currently, we manually stop and start the chess clock when displaying the resign and claim draw dialogs, in order to avoid displaying the pause game overlay.  This is risky since it avoids setting ChessGame.is_paused, which no longer means what it ought to mean.  Decouple the state of this variable from the display of the pause game overlay to avoid resulting bugs (e.g. the chess engine may make a move even while these dialogs are displayed).