Instead of using the "just_audio_windows" and "just_audio_mpv" packages
for Windows and Linux, we are now using "just_audio_media_kit", so that
the audio player is working reliable on these platforms.
Note: We have to adjust the Flatpak, to include the dependencies which
are required on Linux.
Instead of downloading the Flatpak files from GitHub in the
"app.feeddeck.feeddeck.yml" file, we are now adding the files to the
archive we create during the release process.
