De-duplicate Flatpak build setup

[leamas]

Is your feature request related to a problem? Please describe.
The official Flatpak packaging which creates packages users can download lives at flathub. We also maintain a Flatpak setup in the flatpak/ directory. This includes manifest, patches and the shared_modules git submodule.

As of today, the flatpak/ directory represents a duplicated effort which by definition means more work and poor quality.

Describe the solution you'd like
The main benefit of current setup is make -f ../flatpak/Makefile build which builds and installs a local Flatpak build. This interface is sound and should be kept as is.

Instead of using local more or less correct copies of manifest, patches and shared_modules we should use the flathub sources and build from those. This will include some minor patching to the manifest, symlinks and using the shared_modules git submodule.

The main thing is making the flathub setup a submodule in the flatpak/ directory.

Additional Context
We are now entering the beta cycle, and beta packages are created usng flathub. Thus, no hurry.