Add a Flatpak version and publish it on Flathub

[Kazevic]

Is your feature request related to a problem? Please describe.
Having to download the AppImage file every time a new version is released is not very convenient, and AppImages are self-contained, so libraries aren't shared between apps, therefore taking up more storage space.

Describe the solution you'd like
Making a Flatpak version of imFIle and publishing it on Flathub would solve both issues while still having it available on basically all distros, so I believe it wouldn't be much more of a headache to support it.

Describe alternatives you've considered
Distributing RPM/DEB packages would be harder to maintain and support fewer distros, and you can work around the access to files in the Flatpak version, so it isn't necessary.

Additional context
This might be useful: https://docs.flatpak.org/en/latest/first-build.html