Refactored FitsImageCanvas classes

	modified:   ginga/BaseImage.py
	modified:   ginga/FitsImageCanvas.py
	modified:   ginga/cairow/FitsImageCanvasTypesCairo.py
	modified:   ginga/qtw/FitsImageCanvasTypesQt.py

- base class now handles properly non-gui aspects of the forms
- graphics items are defined by the base class item + platform specific
    mixin class

- fixes an issue in Cuts plugin where the cut was not moved from the
    middle of the line