Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions

* browser: px to rem, added background size definitions, removed unnecesary js function to show lateral panel
* added @2x assests for browser (i've changed also some normal icons crop-area to match 30x30px)

Please @benfrancis @timdream @rexboy7 @jmcanterafonseca take a look