Restartless add-on

[Lekensteyn]

Restartless add-ons are easier to test an update
https://developer.mozilla.org/en-US/Add-ons/How_to_convert_an_overlay_extension_to_restartless

According to @Rob--W (who gave a quick look!) it should be possible for this extension.

[spasche]

Good to know that it's possible now.

I won't have time to implement it, but pull requests are welcome ;-)