Fix SkeletonEditor's methods to adapt new animation and skeleton system

[TokageItLab]

#53689 and #53765 make big changed the methods of Skeleton and some SkeletonEditor's functions has broken, so I'll fix that.

[TokageItLab]

Fixed bugs and refactored. However, the animation track insertion is still broken. To fix that, a new UI needs to be generated, so I will send an additional PR later.

The previous EditorProperty in the Inspector did not have a reference to the Skeleton's property directly, but it has been fixed. Instead, Rotation UI is now a quaternion, but the same problem is occurring with RotationTrack in Animation.

Quaternions are difficult to manipulate in the Inspector, so that problem needs to be fixed. Since @reduz is planning to implement rotation modes other than just YXZ Euler and Quaternion in the future, related to that, we are discussing whether we need to implement an EditorPropertyRotation which will replace EditorPropertyQuaternion for the interface.

There is also a problem with global scaling changing the Rotation, which needs to be addressed in the future, for example, such as #53684.

[lyuma]

My preference is to have Bone Transform visible by default, since the user selected the bone to see its transform.

	pose_editor->set_visible(false);

I would also like the button for "apply pose to rest" to be moved, since it is now more important than before, but far away from the Rest matrix in the inspector that it would modify.

[TokageItLab]

Maybe, what @lyuma means is section->unfold(). I agree with that and added one line to fix that.

[akien-mga]

Thanks!