flatpak: update files following names change + Python 3 update: - `build_manifest.py` has been fixed to work with recent SàT/Libervia - filenames/scripts have been udpated to reflect project name change - installation now uses `requirements.txt` when dev version is requested - there are now 3 types of commands wrapper: * `libervia_wrapper.py` uses `pb` bridge, starts backend before frontend, and stops it when frontend is stopped. It's used by `Libervia Desktop` (Cagou) * `libervia_wrapper-dbus.py` uses `dbus` bridge, starts backend before frontend, and stops it when frontend is stopped. It's used for `Libervia TUI` (Primitivus) * `libervia_wrapper-dbus.py` uses `dbus` bridge, starts backend if necessary and doesn't stop it (to avoid waiting for backend start next time). It's used by `Libervia CLI` (jp).

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