Add a possibility to specify whether a KGamePopupItem should hide itself when clicked.

Before this change it was always hidden on click.
Now I added setHideOnMouseClick(bool) function which can be used to toggle this behaviour.

This can be useful in e.g. kdiamond (for its "Game paused" message) and in an upcoming patch for kmines from Gnushi.

CC-ing kde-games-devel in case some other games might find this addition useful.