Rework subscription so that we can use it for client and server.
Move source and source-output to client to allow client provided
sources. Still needs some work but registration seems to work partly.
Rework DBUS API: move CreateSourceOutput to Client1 interface, remove
Add/RemoveProvider and Device1 interface. Rework SourceOutput1 to
allow for reconfigure.
Add a client to test v4l2 source.