fix #9: import as multimaterial meshes
with help from 'bmesh' module
mesh 'unsupported' properties moved to Mesh.xray custom property
export meshes and materials
fix: AV in ActorEditor when reimport stalker_do_balon_4.object
fix: wrong uv-map vertex indices
export smoothing groups
fix #8: export to .object file

export meshes, surfaces(materials), uv-maps, smoothing groups