inVRs

in the current implementation the SystemCore depends on OpenSG as for example the threading is implemented using OpenSG threads. In order to use a different scenegraph implementation (e.g. OpenSceneGraph) these library specific types needs to be extracted.

1. use boost for threading, locking,...
2. write a wrapper around the OpenSG specific classes, which can also be implemented e.g. using OpenSceneGraphs OpenThreads and decide during compile-time which implementation should be used
3. like b but this time, decide at runtime (i.e. during initialization)