Import scenes as AnimationLibrary

[reduz]

Added the ability to import scenes as AnimationLibrary

• Completes implementation of Implement AnimationLibrary resource and respective importer godot-proposals#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.

Screens of usage:

Simply choose any scene file and change the import type from Scene to AnimationLibrary:

Then, the resulting file can be imported into an existing animation player as a library:

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.

Bonus: A couple of bugs fixed in the advanced import settings regarding to option visibility.

bugsquad edit:

Fixes: #60060

[fire]

reduz/godot@animation-library-import...V-Sekai:animation-library-gltf

I noticed some odd points.

1. preview pre import always show the meshes
2. In my branch when I click on the gltf on the file system it changes the import type from animation library back to scene (godot filesytem dock).
3. Added support for the IMPORT_DISCARD_MESHES_AND_MATERIALS flag to gltf import but preimport doesn't know we want to discard the animation.

[fire]

See the comments in the post before.

The filesystem swap isn't blocking work on animation retargeting and since @akien-mga and others will be away for a holiday I'll approve. In my testing the scene does seem to be usable as a resource that can be loaded from an animation player.

[reduz]

reduz/godot@animation-library-import...V-Sekai:animation-library-gltf

I noticed some odd points.

1. preview pre import always show the meshes

2. In my branch when I click on the gltf on the file system it changes the import type from animation library back to scene (godot filesytem dock).

3. Added support for the IMPORT_DISCARD_MESHES_AND_MATERIALS flag to gltf import but preimport doesn't know we want to discard the animation.

Should be working now, this happened if you click re-import in the advanced dialog.

[fire]

The change to provide the right class looks correct. Not able to compile quickly.

[akien-mga]

Code looks good.

It seems a bit weird/uncommon for the Godot codebase to have two different singletons for instances of the same class, but I see how it's convenient here.

[reduz]

@akien-mga There are a couple of cases of the same importer for multiple classes (like LayeredTexture) using a single class, but the singleton is indeed strange. I cant think of a way to better workaround it better to be honest.

[TokageItLab]

I tested, import is working correctly. Possibly we may need to change later on the dealing with the RESET Track (Context: #60093), but that is no reason to block this PR.

[akien-mga]

Thanks!