overview: removed hard-wired thumbnail frame
overview thumbnails: improve stylesheet

- replace hard baked frames with css properties
- adjust position of close buttons and reassure action with
  specific :hover state for the close button
- avoid using alpha for normal state (feels washed out). Raise
  the thumbail on :hover and sunk it for :active

The thing missing is thumbnails still having the shadow baked in.
For this to work, the web thumnails need to remain flat.

https://bugzilla.gnome.org/show_bug.cgi?id=723950
overview: set the thumbnmail size correctly

https://bugzilla.gnome.org/show_bug.cgi?id=723950