Improving tab previews

Improvements:

* title of the page is removed since it is visible on a tab.
* widget now has a smooth 3d look'n'feel with Oxygen thanks to raisen frame
* widget has rouned corners in all the styles
* url of the page is elided
* with compositing tab previews will be shown semitransparent with 25% alpha
by default, witch can be setted to be optional later
* code of tab preview is now separated from tab bar so it is more explicit

REVIEWED BY: Johannes, Andrea, Pierre


REVIEW: 101848