gcode sends too fast

[i-make-robots]

Describe the bug
GCode can be sent from the app to a marlin device too fast. messages might be lost, out of order, or garbled by transmit failure.
There is no receipt system. There is no system to queue, resend, and CRC check messages.

To Reproduce
Steps to reproduce the behavior:

1. Make some gcode
2. connect to the robot
3. send too much gcode too fast.

Expected behavior
Marlin sends "OK" when it is ready for more. do not send more until OK received.
Problem is already well-solved in Makelangelo-software and in RO2.
Implement the solution from there.

Probably requires #234 first.

[i-make-robots]

Presently gcode sending is controlled by the speed of the EditorPanel. rate limiting is a little better, but still not a thorough solution.