Flatpak / Flathub release

[Stefterv]

I created a flatpak configuration file for a release on Flathub.

• Set up the mime type and application icon
• Check if the sandboxing is set up correctly
• Revisit the mime-types in the snap configuration so both can be synced
• Add gradle tasks for local builds/installs
• Create an automation that will dynamically generate the flat hub file and create a PR on the target repo

To consider:

• Should we add a single file to the releases so people can install the flatpak version without flathub?

Closes #890

[SableRaf]

@Stefterv will there be a new PR for this one?

[Stefterv]

Yes, I worked on my main branch for the GitHub Actions to work as expected but I'll be creating a new branch later