Patch to make common octave_dock_widget class

[Richard Crozier]

I have added a patch (#7943) which creates a class octave_dock_widget 
from which the other classes previously inherited from QDockWidget 
directly are now inheirited instead.

I asked a while ago if this would be any use and the consensus was yes, 
it might be. I finally got around to doing it.

Richard

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