Proper handling when adding mimic motors

[hwiedPro]

Describe the bug Creating a mimic joint provides the optimon to create a mimic motor. This writes the mimic motor to the corresponding link (like in the old definition) and not to the corresponding motor.

Expected behavior When add mimic motor is checked. We should have a look if there is already a motor: If yes write the mimic parameters to this one. If not, create a new one with the mimic parameters that either copies maxEffort & maxSpeed or asks for those parameters.