Preliminary use of query mask flags.
Extract the SelectionListener interface to a separate file.
Add missing files from previous commit.
Clear the selection whenever the selection type is changed.
Add PlayingState::ExecuteAction(const game::PlayerAction&).
Deny direct access to the listener list.
Modify the code since now the listeners() method returns a copy of the list.
Modify the code since now the listeners() method returns a copy of the list.
Allow a human player to perform a PLACE_PIECE action.
Make the BoardView react to PLACE_PIECE actions.