Multilayer PCB not working with Protel file extensions

[leonhardpetzel]

When loading gerber zip files with Protel file extensions, then the inner layers of 4 layer PCBs are discarded. Using KiCad, the inner files have the extensions ".g2" and ".g3". Those extensions are not present in the extensions lists in "do_optimize.py" and "PcbExport.py". Adding the names to extensions and extensions_to_names seems to fix this issue.