Allow button presses to queue the action to be performed instead of doing it directly.

In some applications if an action is performed, it will invoke a dialog or open another window. If that action has been triggered from a MenuPopup (so in this case the button will be a MenuButton subclass of Button) the focus will be stolen from the invoked dialog back to the main window by the closing of the MenuPopup. The result is that the user must click another time on the dialog. So, one solution to this dilemma is to queue the action to be performed "later" on the event queue. This allows the menu popup window to close before the action is invoked and therefore the focus can be shifted properly.

So, add a property to Button, which will be set by MenuButton to queue the action instead of performing it directly inside "Button.press()".

This is a Proof-Of-Concept since I haven't yet figured out how to get this to work properly.