Flatpak CI Build

The current build artifacts have the disadvantage that one needs to manually install all runtime dependencies on the system manually. With this the CI creates a flatpak file that can be used on all linux systems with flatpak installed with no other actions needed. This allows easy use of experimental builds on any linux distribution.

The Flatpak manifest is inspired by the current OpenMW version in flathub (Github).
One of the changes compared to the flathub versions is a new app ID. This allows installing the release version on flathub and the experimental build from the CI at the same time.

I had one issue on my fork when implementing this though. Flatpak builds all dependencies of the applications from source. When a library version has been built once its result will be cached in the .flatpak-builder folder. However, When the cache is completely empty, the pipeline doesn't seem to be able to finish in the 3h timeout gitlabs shared runners have. I am unsure what a solution to this could be. On my fork i solved this issue by not using the shared runners but my own.