Overriding process_button_event()

customcanvas.C shows a simple example of handling button clicks. This is done by having the implementation object override process_button_event(). The implementation object falls back to the overridden method if it chooses not to process the button event; otherwise it returns true.

customcanvas.C responds to a pointer button #1 click by adjusting the custom element's metrics by invoking update() that it inherits from x::w::canvasObj::implObj.