Learn if a button is pressed (or released).
The Area.Pressed attribute of a gadget is triggered
by some user action, depending on the input mode:
RelVerify:-
Set when lmb is pressed. Cleared when lmb is released and the mouse
is still over the gadget (otherwise it will be cleared too, but without
triggering a notification event).
Immediate:-
Undefined, use Area.Selected for this.
Toggle:-
Undefined, use Area.Selected for this.
Waiting for Area.Pressed getting False is the usual
way to react on button gadgets.