Abort heating

[peteruithoven]

Currently there is no way to make the printer stop heating. It will continue doing other commands only after heating is finished.

This happens because the printer only sends a next command to the printer when it receives an ok from the printer. When you send marlin firmware commands like an M109 it will send an ok when it's done heating.
We can only abort this by sending another command immediately.
We should be very careful to make sure this only happens when needed because we don't want it to interrupt prints accidentally.

When you're in a controlled setup where a lot of prints are made continuously you could replace the M109 with an M104 in the start gcode and make the preheat temperature the same as the printing temperature.