Add development flatpak manifest

[ayykamp]

I added a new flatpak manifest to replace the non functional one that was in the repo. To test:

flatpak install org.flatpak.Builder
flatpak run org.flatpak.Builder --repo repo --force-clean --install-deps-from flathub --user --install build build-aux/de.feschber.LanMouse.yml
flatpak run de.feschber.LanMouse

Note that to submit to flathub there will need to be some further work, because their build process does not allow network access. See flatpak-cargo-generator or cargo-vendor.