Experimental, compiles and runs but crashes in some exit scenarios.
Add missing files.
Fix crash when multiple scenewidgets were closed.  BtOgre was destroying resources each time.
Remove unused code.  Move the cleanup of global resources used by the PhysicsEngine to PhysicsManager.
Include string header for gcc.
Fix memory leak when multiple documents in 3D edit.  Support multiple physics engine per document.
Merge branch 'master' into mouse-picking-physics
Remove files no longer used, reverting to OEngine.