VTXNORMS chunk for ActorX (vertex normals)

[floxay]

Blender plugins that support this chunk:

• blender3d_import_psk_psa by Befzz (https://github.com/Befzz/blender3d_import_psk_psa/pull/69)
• io_scene_psk_psa by DarklightGames (https://github.com/DarklightGames/io_scene_psk_psa/pull/17)

[gildor2]

This won't work with Unreal engine itself. The code in UE3 doesn't check chunk names, it just assumes particular chunk order. So, new chunks should be always added to the end.