This is a cleaner way to interface with the remote pipewire daemon.
The WpRemote base class can be subclassed also for interfacing
with other daemons (hardware-specific managers, etc)

and also implement
* global-added/removed signals
* a foreach function to iterate through globals
* registering modules and factories with the same key

After discussing things at the AGL May 2019 F2F meeting
and reflecting on the initial design of WirePlumber,
it became clear that it needed a fresh start.