IK as an animation blend node

[fire]

Posting a different version of the IK solver immediately after GDC. Still a bit tired.

• Provide UIUX
• Restore functionality
• Restore editor widgets for constraints

Alternative version of #70887 that takes feedback from GDC design review from Mizizi and others. Also had design feedback from Eron, Tokage, Lyuma and Saracen.

Also tries to Fix: godotengine/godot-proposals#6039 Will write a proposal for this alternative form when I'm able to.

Solving for targets & root motion lets roots move freely. Need to handle FK animations & let artists focus on specific parts like hand grabs & foot plants w/o configuring the entire skeleton.

1. input animation
2. target animation poses
3. a comfortable animation pose to pull towards
4. filterable tracks
5. Inspector settings

[fire]

Closing for now. Salvageable.