Splitting up of KWin's global D-Bus interface

Two new interfaces are introduced:
* org.kde.kwin.Compositing
* org.kde.kwin.Effects

The Compositing interface is generated from scriptable elements on the
KWin::Compositor class and the Compositor is exported as /Compositor.
It provides the general Compositing related D-Bus methods like whether
the compositor is active and toggling and so on.

The Effects interface is generated from scriptable elements on the
KWin::EffectsHandlerImpl class and the instance is exported as /Effects.
It provides all the effects related D-Bus methods like loading an effect
or the list of all effects.

This removes the need to have all these methods provided on the global
org.kde.KWin interface. For backwards compatibility they are kept, but
no longer provided by the Workspace class. Instead a new DBusInterface
is generated which wrapps the calls and delegates it to one of our three
related Singleton objects:
* Workspace
* Compositor
* EffectsHandlerImpl