Make unstore AnimationLibrary if AnimationTree is assigned AnimationPlayer

[TokageItLab]

Reported a problem regarding when to enable EditableChildren from @Mickeon.

AnimationTree should not store libraries in .tres when it has an AnimationPlayer, as the AnimationLibrary resource may be stored duplicately.

Since this fix concerns of data lost, I would appreciate if someone could please carefully test this fix.

Bugsquad edit:

• Should solve AnimationTree's own libraries are duplicated when borrowed from an inherited (or editable) scene's AnimationPlayer #85602

[Mickeon]

It has been mentioned in RocketChat that this issue stems from the fact that you cannot reference sub-resources from another scene (at least not yet). While the solution here looks about right, it may be worth adding a comment on why this is done because it does look a bit out of place.

[fire]

@SaracenOne You worked with referencing sub-resources from another scene too, want to take a look?

[akien-mga]

Worth noting that this is pretty much reviving #82750. I do think that's a much safer ad hoc fix suitable for 4.2.x than #84168 (which might be the better/more thorough fix, but seems risky for a cherrypick).

[SaracenOne]

Yeah, upon revisting this, I think this approach for fixing this one specific bug is still relevant, though we will still need another PR to fix other resource duplication bugs, even mine or an alternative fix.

[SaracenOne]

Since this is revisting a previous PR, I think this is okay to merge. That being said, the other PR is still indeed very much relevant since it fixes other crucial things that this PR does not address. I agree with @akien-mga my other PR is risky, but so far I've not been able to come up with an alternative approach for fixing the broader bug, which I'm hoping more people could review it. I know @lyuma had some thoughts on the potential implications of preserving foreign resource references, so I might see if I can chat to him about it.

[YuriSizov]

Thanks!

[YuriSizov]

Cherry-picked for 4.2.2.