Mute sound when playing tests.
Start selenium under Xvfb when using Linux
Add a DISABLE_XFVB env var
Add documentation about XFVB support
Merge pull request #389 from ametaireau/headless-tests

Headless tests on Linux. r=Standard8
Restyle the gear menu to match the mockups
Add comments and refactor some CSS rules
Put back sidebar template in place.

This was useful to debug the HTML / CSS panel, less useful to land it on
master.
Fix the Xfvb rule
Remove Xfvb setup from the travis config
Merge pull request #391 from ametaireau/fix-travis

Remove Xfvb setup from the travis config. r=Standard8