cura: properly working dual extrusion plugin

No longer relies on custom gcode to function and is mostly safe for
human consumption.

 * Injects M109 commands before unspooling a newly activated hotend to
   ensure printing temperature is hit. This will occur over the priming
   tower if it is enabled and eliminates oozing over the print.

 * Unconditionally cools inactive hotend even if the estimated layer
   time is less than the preheat time. The hotend instead reheats once
   the active hotend is up to temperature, preventing some ooze and
   potential heat creep at the expense of some print time.

 * The extruder can be raised by a configurable amount while waiting for
   the active extruder to fully reheat after tool change.