Code Cleanup for 2.1

I as a code writer would like to see a clean source code hierarchy that is easy to approach and makes intuitively sense. Now that Qt 3D Studio is built on top of Qt, we should minimize OS dependant code structures, file structures etc. Also we still have left overs from past code porting efforts that should be cleaned up.

This user story collects all tasks we aim to clean up in Qt 3D Studio 2.1 release in the authoring tool, in the legacy runtime and in the current runtime.