You receive one mousedown event per click with a mouse-state structure pointed to by parameter 1.
By accepting this you eat the click, otherwise your parent gets it. You will not receive drag and
mouse up messages if you do not accept the down message.
Handling this message consumes the mouse click, not handling it passes it to the next widget.
You can act 'transparent' as a window by never handling moues clicks to certain areas.
Dispatching: Up chain NOTE: Technically this is direct dispatched, but the widgets library will ship
it to each widget until one consumes the click, making it effectively "up chain".
Param 1: A pointer to an XPMouseState_t containing the mouse status.