output: disable output autoconfig if we got a wlr-output-management client

[emersion]

When a daemon such as kanshi is connected, we don't want to auto-configure the output in such a way that the daemon would need to undo. Fix this by disabling output auto-configuration while a wlr-output-management client is connected.

This has two outstanding issues:

• Some wlr-output-management clients just monitor changes without being responsible for configuring outputs (e.g. wdisplays running in the background).
• kanshi will not set fields unless the user has explicitly configured them, so we can end up with an enabled output without a mode.