Make theme HiDPI-compliant

[fofajardo]

This way, the theme's images won't look stretched even if our screen scale goes higher.

• Recreate toolbar images from SVG
• Recreate tab image using CSS (as close as possible or SVG-ize it)
• Recreate folder, bookmark, RSS, and other images from SVG

[fofajardo]

Assets for this were stored in a separate, local branch.