Make panel close control honour the state of a quit button (if any)

[RobertoBozzolo]

This suggestion comes from this discussion: basically, it seems strange to me that I may have a disabled quit button and the close control (the red cross in the upper right corner of the panel) which is active at the same time.

It seems to me that the close control should honour the state of an existing quit button declared in Close control panel attribute. At present the only solution is to test the state of the button within the button callback itself.

If this idea is not accepted, at least a note should be added to the documentation, e.g. in ATTR_CLOSE_CTRL help page, specifying that the callback is fired regardless the state of the control.