Synchronize scale value

The kwinft_output_management_unstable_v1 protocol only allows to set the logical size but no scale value.

That's fine from the viewpoint of the compositor. But the KDisplay KCM at least still needs to communicate to the daemon the current scale value. We could do this for now via the control file like we do for the replication source.

Long-term one has to decide if this is enough or if we want:

• some synchronization method through Disman,
• add metadata to the kwinft_output_management_unstable_v1 and kwinft_output_device_unstable_v1 protocols that don't influence the compositor state but give other clients more information about the current configuration.