This fixes a regression found in TestSceneQPainter for restarting the
Compositor. The internal clients were never added to the Scene.

We cannot just call setupCompositing because there is a phase during
startup where setupCompositing already passes but the Compositor is not
yet fully initialized. Thus it could happen that it's called twice which
makes the Scene assert.

To solve that setupCompositing and finishCompositing are now overriden
in ShellClient and track whether they successfully called
setupCompositing.