This can be used to make our backends more multi-purpose. At the moment,
new outputs are enabled by default, but it makes sense to do otherwise.

For example, if an output is disabled by default, it would be possible
to delegate initial output configuration to layer above, to kwin.

In long term, the drm backend would need to scan connectors, create an
Output for every one of them, kwin sees new outputs and tries to apply
the initial output configuration, which includes the enabled status.