Some VFS fixes: attempt to load .pak and .wad, load png fallback even if game does not support png, do not load engine path twice

• load .pak and .wad files, fix #139 (closed)
• add .pakdir support for free (but that's not yet useful before map compilers handle it)
• fix loading PNG fallbacks (notex, etc.) when the game does not support PNG, fix #141 (closed)
• do not load engine path twice when home path is engine path (happens when the game does not have home path)
• properly list wads as texture directories, with working tree, per wad texture listing and wad name in texture window title