Event handling updated

	modified:   ginga/FitsImageCanvas.py
	modified:   ginga/Mixins.py
	modified:   ginga/RGBMap.py
	modified:   ginga/gtkw/GingaGtk.py
	modified:   ginga/gtkw/PluginManagerGtk.py
	modified:   ginga/gtkw/plugins/Catalogs.py
	modified:   ginga/gtkw/plugins/Histogram.py
	modified:   ginga/gtkw/plugins/Pick.py
	modified:   ginga/gtkw/plugins/PixTable.py
	modified:   ginga/gtkw/plugins/Zoom.py
	modified:   ginga/misc/plugins/CutsBase.py
	modified:   ginga/qtw/GingaQt.py
	modified:   ginga/qtw/PluginManagerQt.py
	modified:   ginga/qtw/plugins/Catalogs.py
	modified:   ginga/qtw/plugins/Histogram.py
	modified:   ginga/qtw/plugins/Pan.py
	modified:   ginga/qtw/plugins/Pick.py
	modified:   ginga/qtw/plugins/PixTable.py
	modified:   ginga/qtw/plugins/Zoom.py

- Now provides default binding of color map warping with second mouse
    button
- Now provides default binding of porportional pan to first mouse button
- Both are disabled if a plugin has taken those bindings