at the end of the menu only.
Provided a general method for a menu function to detect exiting by a timeout.
When exiting, the menu function is called with a global variable menuExiting set.
Some errorneously removed functions were reverted.
Implemented bed skew calibration by matching a precise physical model
to the measured data using the least squares method.
Rewrote handling of the command buffer to preserve memory
and allow pushing the commands to the front of the queue.
