Tracking reticle for Arm IK

[Levi-Lesches]

In addition to simply printing out where the arm is planning to move, it might be helpful to have an on-screen visual in the form of a reticle that "guides" the gripper. This way, the operator can intuit the capacity of the arm to move to that location and how long it'll take. This might be a little hard because we support 3D, so we might go with smaller/bigger for farther/closer.

This can be implemented as a Align widget that normalizes the coordinates to be between [-1, 1] to render itself on top of the camera feed.

[Levi-Lesches]

Implemented by #147