Added the ability to import scenes as AnimationLibrary
* Completes implementation of https://github.com/godotengine/godot-proposals/issues/4296
* Helps if you want to export animations to a separate file (say a GLTF) to avoid re-importing/exporting them every time the model changes.
* Helps if you simply want to have animations using a dummy model, which can be shared across multiple models.
Creates a secondary scene importer used only for animations.
**NOTE**: A new flag for scene importer: EditorSceneFormatImporter.IMPORT_DISCARD_MESHES_AND_MATERIALS has been added, to hint importers that they should skip meshes and animations (and hence make importing faster). It is not implemented in any importer yet, this should be done in a separate PR.
