IO enhancements
Made selectable in exporters which renderer's actors to export. Fixed invalid material file path in OBJ files (absolute path was written instead of relative). Made it possible to read/write file comments in STL and OBJ files (to able to store metadata, such as color or coordinate system name).
Required for https://issues.slicer.org/view.php?id=4445 and https://issues.slicer.org/view.php?id=4408.