Add Support for Zen Browser (#335)

[DanWolfstone]

This PR Includes support for the Zen browser flatpak (firefox based)

I mimicked the implementation of the firefox flatpak.

#335

[giatmanos]

I have visually reviewed and they seem correct to me. I have not tested these changes myself.

[TarsVerschelde]

I've tested this on my system (linux mint LMDE where I replaced cinnamon with gnome) and it works just fine.

There are some minor issues:

• When you start the webapp for the first time, it goes through the initial setup of the zen browser again.
• Additionally it shows some overlay buttons that dissappear for a while once you've clicked them.
• By default the zen browser does not show the title bar but in the webapp it does, which feels a bit awkward.

Some of these issues may be issues of zen browser rather than web app manager.

Regardless, none of these minor issues should withhold this PR from being merged as, in my opinion, it is better to have the feature in a rough version than to not have the feature at all.