Optimise old mesh files on load

Older versions of Qt 3D Studio have saved undo information as part of mesh files, this is unnecessary as it just increases the size of the .mesh file and undo is expected to be available in the DCC tools artists use.

Now that we no longer do this in 2.0 version, it would be good to strip out the unnecessary versions of meshes from older projects when user next time opens the project. This means: check if there is versions of meshes in the files, delete all but the latest one.