Re-write GUI in Qt and try and make it pluggable in Glue

I'd like to try and work on re-writing the GUI in Qt (the matplotlib one kind of sucks for a number of reasons) and at the same time I think it would be nice to explore how we can do this and make it usable in glue at the same time.

cc @keflavich @ChrisBeaumont 
@keflavich @astrofrog I think this line is causing the traceback adam is seeing. I refactored PVSliceWidget a few weeks ago while working on ginga integration, such that it expects `image_client=self.image.client` instead of `image_widget=self.image`
This line is