Mesh copy support

Currently, the C++ default copy constructors handle copying a mesh; this seems problematic in the ResourceManager, which keeps the mesh properties as vectors of raw pointers, so we have a duplicate ownership situation after the copy.

I would change the code to clone the properties (assuming valid copy constructors for property types).

This is related to https://www.graphics.rwth-aachen.de:9000/OpenFlipper-Free/OpenFlipper-Free/issues/143