Unload self-loaded stylesheets when extension is unloaded?

[eyalroz]

The extension currently loads/injects two stylesheets of its own into displayed messages: quotebar.css and direction-autodetection.css. When the extension is unloaded while a message is displayed, and without the app going down too, perhaps should probably unload those stylesheets. Possibly even unset all of the attributes we set before.