Add Zen browser support

[dtrossiter]

Does not currently support the Zen browser (firefox fork) that is fairly new. Please add this support when it makes sense and is convenient.

[TarsVerschelde]

If you cannot wait for the official support, you can already use the files from the pending pull request (#336). I've tested them on my machine and it works.

The files can be found here: https://github.com/DanWolfstone/webapp-manager/tree/patch-1/usr/lib/webapp-manager

These files will have to replace the files in this folder: /lib/webapp-manager

This swap is, of course, at your own risk ;)