Grasp and release virtual items

[i-make-robots]

Is your feature request related to a problem? Please describe.
Virtual grippers have no way to pick up, move, and release objects, making it hard to simulate activities.
Virtual arms have no way to run an Automatic Tool Changer (ATC), effectively picking up a tool that could then also grab something else.

Describe the solution you'd like
AttachmentPoint extends Pose.

• Has some kind of collision radius.
• When toggled on, any Pose in the collision radius is moved in the scene tree to be a child of the attachment point.
• When toggled off, the reverse occurs.

arm end effector becomes an AttachmentPoint. MarlinRobotArm would need to make attach/detach controls available in the GUI?
Tools that pick things up would also need to have AttachmentPoints. When an arm has a tool with an AttachmentPoint, how do those GUIs stack?

How does this work with analog grippers that only close a little bit?

How does this work with items that should not be moved from their location in the scene graph? Would a very strong rubber band be better?

[i-make-robots]

• illustrate the "grab zone" of the attachment point.
• make the servo action of a MarlinRobotArm cause the tool (if any) to attach/release.

[i-make-robots]

The LimbSolver.target is a Pose, which becomes attached. After that moving the target causes a relative motion.
Works as intended and yet is also wrong. Hm...

[i-make-robots]

54cb5ee improved attach and release does not grab empty poses (aka the target)