RenderTrigger is a class for managing render requests for a scene (or parts of it)
One can add subtrees of a scene (SceneGraphComponents and their children) to be watched
for changes. On the other side one can add Viewers on which render() will be called
for any change that is reported. The renderTrigger knows its viewer.
If one adds only Viewers and no scene it can be used to simply dispatch render requests
to several viewers (via the forceRender() method).
Note: in the toolSystem when a toolEvent was of the type systemTime, the redertrigger is called to render
TODO: fix problems maybe use Proxy (remove/add doesn't work how it should)