cura: dual extrusion improvements

Plugin is even more fit for human consumption, but the code is growing
less and less fit (global variables, static gcode, the agony)

 * Disable fan when reheating. Keeping the fan on the corner of the
   priming tower made it detach from my bed and reheating is a little
   faster without it off.

 * Restore behavior of not reheating during a very short layer. If
   layers end up short for both hotends, ooze returned and the effect of
   the plugin was negated.

 * Only hop z on the first layer to prevent a blob from forming at the
   edge of the priming tower.