* added filtering out of invalid components/ custom properties:
* includes filtering out of invalid / disabled components as defined in bevy_components, components_meta etc
* completely changed handling of "unique" (non blueprint) objects by copying them instead of moving/renaming them & manipulating their custom properties : much cleaner, much simpler !
* disabled default gltf export of optimised animations
* added tests !
* closes#139
* closes#141
* closes#142
* closes#146