Misc bug fixes

	modified:   ginga/Bindings.py
	modified:   ginga/FitsImage.py
	modified:   ginga/gtkw/plugins/Thumbs.py
	modified:   ginga/misc/plugins/ThumbsBase.py
	modified:   ginga/qtw/plugins/Thumbs.py

- Fixed bug in drag panning (aka proportional pan) where image would not
    drag smoothly
- Fixed thumbnail generation to regenerate a thumb immediately (instead of
    after a delay) and also to use the current cut levels
- Added a "Clear" button to the Thumbs plugin to clear the set of thumbnails