Provide a Flatpak package.

[RokeJulianLockhart]

My rationale is available at https://github.com/safing/portmaster-packaging/issues/43#issue-956312378#:~:text=It%20shall%20allow,to%20support%20this. I am thankful for any assistance.

[4ch1m]

Please provide a pull-request containing all the necessary adjustments.
If the implementation has a "small footprint" (and is easy to maintain) then I'll consider to add it permanently.
Thank you.