Introduce a callback in `outputs::Outputs` that calls on every `wl_output.done`, and a dummy consumer in `ui_manager`.
This is sufficient to detect display height changes.
The manager is used for sizing the layer surface. It promises never to exceed half the output height.
The selection of the current layout is not being done here, leading to worse behaviour in 1:1 scaling.
In the future, it could be used for sizing the keyboard itself and the suggestion box, as well as decide which layout to use, because layouts should have some sizing hints.
