Extend gaze controller

The low level gaze controller currently computes the desired head joint angles based on the "atan2-IK". Then, these target joint angles are just given to a position controller.

This approach has several limitations:

• The velocity of the head movement cannot be controlled. Based on the importance of the gaze target, this should be variable.
• The joints are controlled individually. One would expect a holistic behavior such that the velocity of both joints are coupled.