Flatpak manifest for Rclone Shuttle

Build instructions

• In case of Rust dependency changes, get flatpak-cargo-generator.py (not included here because license is unknown), install dependencies, then execute flatpak-cargo-generator.py /path/to/code/repo/Cargo.lock -o /path/to/manifest/repo/cargo-sources.json.
• In case of Go dependency changes in Rclone, use flatpak-go-mod to regenerate go-sources.yml.
• Run ./flatpak-build.sh to do a local test build. This script is not used by Flathub's build system.