Implements https://github.com/godotengine/godot-proposals/issues/4096
* Nodes can be marked unique to the scene in the editor (or via code).
* Unique nodes can be accessed via the **%** prefix at any point in the path. From that point in the path (depending on whether the scene of the path is), the unique node will be fetched.
* Implementation is very optimal, as these nodes are cached.
unzReadCurrentFile(3) returns number of bytes read, so the current code only processes the file if no bytes were read (UNZ_OK is a #define equal to 0).
I've altered the break to occur only on unsuccessful read, (when unzReadCurrentFile(3) returns less than zero), and added an error message for when an unsuccessful read occurs.
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.
* Instead of containing single animations, AnimationPlayer now contains libraries.
* Libraries, in turn, contain the animations.
This paves the way for implementing the possibility of importing scenes as animation libraries, finally allowing to import animations separate from the 3D models.
Missing (will be done on separate PRs):
* Make it possible to import scenes (dae/fbx/gltf) as animation libraries.
* Make it possible for AnimationTree to import animation libraries on its own, so it does not rely on AnimationPlayer for everything.
Both unselected and selected keyframes feature hover feedback.
This currently only affects standard keyframes (i.e. not booleans,
colors or audio tracks which use custom icons).
