Remove bone_pose_updated signal and replace it with the skeleton_updated signal

[TokageItLab]

As has always been the case, the bone_pose_updated signal is fired a crazy many times in a single frame and can cause unusual updates in BoneAttachment. It should never be used, but used only by BoneAttachment.

For safety and performance, make skeleton fire the skeleton_updated signal only once per frame at the moment all modifiers processed. Then, the BoneAttachment uses it.

If the user has been using the bone_pose_updated signal, replace it with the pose_updated signal which is used by some Editors. This signal may also be fired multiple times per frame, but is relatively safe to use since it is not fired during the update process although it does not detect changes made by the Modifier.

If you need a post modification signal for a specific modifier, use the already existing SkeletonModifier3d.modifier_processed signal.

For getting the final pose which will be reflected in the skin, use the skeleton_updated signal that this PR adds.

[lyuma]

Approve in a technical sense. It's going to be a really big performance improvement for rigs with lots of bones and lots of bone attachments.

I think we need to understand the overall design regarding the update order. Also, is it okay from the user perspective for bone attachment to only update once per frame?

I think it might be good to first figure out if the skeleton modifier design changes before merging this.

[TokageItLab]

I think we need to understand the overall design regarding the update order. Also, is it okay from the user perspective for bone attachment to only update once per frame?

I have already agreed with @reduz in the past that if poses during modification by the Modifier are needed, the user must either use the modification_processed signal or create their own SkeletonModifier3D.

From the use case of the BoneAttachment, I conclude for now that it is only important to be consistent with the final visual pose.

[akien-mga]

Thanks!