bridge (dbus): use Tx DBus for backend part of D-Bus bridge:

Due to recent SQLAlchemy integration, Libervia is now using AsyncIO loop exclusively as
main loop, thus GLib's one can't be used anymore (event if it could be in a separate
thread).
Furthermore Python-DBus is known to have design flaws mentioned even in the official
documentation.

Tx DBus is now used to replace Python-DBus, but only for the backend for now, as it will
need some work on the frontend before we can get completely rid of it.

setup.py

@@ -54,6 +54,7 @@ install_requires = [
     'pyyaml',
     'sqlalchemy >= 1.4',
     'aiosqlite',
+    'txdbus'
 ]
 
 extras_require = {