Makes the client's life much easier to identify between multiple remote
outputs. xdg_output is advertising (in later versions) the head name,
but in case we have different plug-ins or multiple remote
outputs created, it would only repeat/advertise the same name
for all (remoting) outputs.
This instead uses a string that uses both the connector and name to
derive a more easier identifier a client can choose from.
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>