Add Flatpak package to Flathub

[Calinou]

I created a fully functional Flatpak package for OpenJazz: https://github.com/Calinou/eu.alister.OpenJazz

I'd like to submit it to Flathub, but there are two things I need for that:

• A new tagged release (as development snapshots are only accepted as a last resort, and the latest commit includes many desirable features such as nearest-neighbor scaling).
• Approval from an OpenJazz maintainer, as this is now required for new applications to be added to Flathub.

[aukondk]

Would like to express a desire for this. OpenJazz is really good on the Steam Deck but to get it running I needed to set up DistroBox with an Ubuntu image to install the required libs. Jazz^2 Resurrection just released a new version with a Flatpak and it was a lot easier to install.