Enhancements to Cuts plugin, partial commit

	modified:   ginga/FitsImageCanvas.py
	modified:   ginga/gtkw/plugins/Cuts.py
Factored out common code of plugins to a base class

	modified:   ginga/gtkw/plugins/Cuts.py
	new file:   ginga/misc/plugins/CutsBase.py
	modified:   ginga/qtw/plugins/Cuts.py
Merge branch 'advcuts'