Preventing prontserve from sending M105 temperature polling requests while the printer is heating up (M109 / no 'ok' received).

The if f >= 0 could be have been accessed even though f had not been
assigned (if float(l) failed)

Improving prontserve's temperature events. Resetting the printer on prontserve estop. Estopping on initial connection by default to kill off stray m109s.

We were still parsing the files twice for this case as the command line parser
was loading them, and the PronterWindow constructor was reloading them before
updating the UI controls.

This is achieved by reworking the logic to look at next move:
if we know the next move will set absolute E if needed, we don't need
to do it for this line. We can thus skip this G-code.