panel: Don't propagate button-release-event in _onCornerClicked()

Since both the hot corner's ClutterGroup and the hot corner's
ClutterRectangle button-release-event is connected to
_onCornerClicked() we must handle it there by returning 'true' to
Clutter or else _onCornerClicked() is called twice which defeats the
HOT_CORNER_ACTIVATION_TIMEOUT logic.

https://bugzilla.gnome.org/show_bug.cgi?id=649427