Make MarlinRobotArm behave like Marlin firmware

[i-make-robots]

Is your feature request related to a problem? Please describe. If MarlinRobotArm can behave like actual Marlin then we gain several benefits:

• accurate time estimation
• real and simulated arms both communicate the same way
• ...so EditorPanel can talk to both
• ...so testing can be automated

Describe the solution you'd like Make MarlinRobotArm behave like Marlin firmware

Describe alternatives you've considered None.

Additional context See com/marginallyclever/makelangelo/plotter/marlinsimulation from makelangelo-software.