* changed in PaintRedirector from Workspace to Compositor to check for compositing

Remove superfluous Compositor checks in events.cpp

Remove superfluous Compositor checks in events.cpp

compositing() ensures that m_compositor is not null.

Make the Compositor a proper Singleton

The Compositor class actually behaves like a Singleton so it should be
one. Therefore three 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

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