Allow engine to force popup of its settings dialog

Allow engine to force popup of its settings dialog

An engine can now force popping up of its settings dialog by sending
'feature done=3' to the GUI. It can clear its option list (e.g. for the
purpose of resending it because of altered settings) by sending
'feature done=0'. Such resending would cause the dialog to be updated
when it was already open, or close it and pop up a new one when the
engine specifies it has to be redesigned by ending the list with
'feature done=2'.