Make pw_remote_connect_fd() not automatically close the provided
fd but let the caller take care of that. This allows us to reuse
the fd in pipewiresrc.
Fixes#155
Make the protocol client connect call async with a callback when it
completes.
Move the connect methods into separate files, add an empty connect
method that will use the screencast portal to get a pipewire fd.
Use the remote intention to get the connect method.
Add some better error reporting.