1) Added parsing of 3MF PrusaSlicer generator semantic version
2) For 3MFs generated by >= "2.4.0-alpha1" and < "2.4.0-alpha3",
remove unreferenced vertices on loading. This should shrink the incorrectly
generated 3MFs back.
3) Added check for an empty mesh, which could have crashed PrusaSlicer on 3MF parsing.