glTF: fix vertex color import + export option tweaks

io_scene_gltf2/__init__.py

io_scene_gltf2/blender/exp/gltf2_blender_export_keys.py

io_scene_gltf2/blender/exp/gltf2_blender_gather_cameras.py

io_scene_gltf2/blender/imp/gltf2_blender_mesh.py

io_scene_gltf2/io/exp/gltf2_io_export.py