Improve UI display of thumbnails

Created by: wilaw

The thumbnail support is a great new addition to the library, however the display using the default control bar can probably be improved.

Some suggestions

1. Lift the thumb vertically away from the scrub track
2. Add a drop shadow to the thumb to lift if from the background video
3. Overlay a seek-point timestamp over the thumb image.
4. Scale the thumbs to be a certain % of the video height (with some reasonable constraints. Tiny thumbs should not be scaled too high on TV screens for example).