Bug fixes for Thumbs and Preferences plugins

	modified:   ginga/gtkw/plugins/Preferences.py
	modified:   ginga/gtkw/plugins/Thumbs.py
	modified:   ginga/qtw/plugins/Preferences.py
	modified:   ginga/qtw/plugins/Thumbs.py

- added "Auto scroll" checkbox to the bottom of the Thumbs plugin gui
  - if checked thumbs window will scroll when new images arrive
  - this can be set by a preference in plugin_Thumbs: "autoScroll: False"
  - default is True
- fixed an issue where the thumbnail was not auto cut
- fixed a bug in the Preferences gui where the auto cuts method widget was
    not updated correctly