Make the Compositor a proper Singleton

The Compositor class actually behaves like a Singleton so it should be
one. Therefore four static methods are added:
* self() to access the Singleton
* createCompositor() to be used by Workspace to create the instance
* isCreated() to have a simple check whether the Singleton is already
created
* compositing() as a shortcut to test whether the compositor has been
created and is active

The isCreated() check is actually required as especially Clients might
be created and trying to access the Compositor before it is setup.