Untouchable control

[gdargaud]

Sometimes I want to complement standard UIR controls with custom graphics on my own. For instance an arrow linking two otherwise normal controls. One way to do those custom graphics is to draw them in a transparent canvas. But then if you place the canvas on top of the controls, it catches the clic events destined for the controls below.

There should be either:

- an attribute for the canvas (or maybe all controls) that basically says 'ignore me and pass the event to the control below me'

- a way for the canvas callback to say 'nope, this event is not for me, just pass it on to the control below me'