Introduce dedicated suspend and resume methods on Compositing interface

The new methods suspend and resume are meant to provide a better way to
influence the current compositing state than toggleCompositing. In
addition an overload setCompositing(bool) is added. The resume method is
implemented in a way that it can be used to try to start the compositor
again in case it failed.

Internally the method suspendResume is dropped as it does the same as
setCompositing just with inverted binary logic and worse name. The
compositingToggled signal is now emitted from within setup and finish to
ensure that especially the compositingToggled(true) signal is only
emitted if the Compositor could start.

Also the updateCompositingBlocking is adjusted to use the new dedicated
suspend and resume methods instead of the toggle method.

REVIEW: 106273