Flatpak packaging for Quod Libet

Add the Flathub repository and install the necessary SDK:

$ make setup

To build the package, run:

$ make BUILD=/path/to/build/dir

This builds both the Flatpak repo (in <build>/repo) and the bundles in dist/. To install the package from the build repo:

$ flatpak remote-add --no-gpg-verify --if-not-exists build /path/to/build/dir/repo
$ flatpak install build io.github.quodlibet.QuodLibet

Updating the Python dependencies

Run

$ make python-modules

to re-generate the modules for the Python dependencies. The generated files (python-*-depends.json) are kept in Git, so this target only needs to be built after changing the list of dependencies or to update the versions.