Improve inheritance function for speed_travel_layer_0
Improve inheritance function for speed_travel_layer_0

This makes the speed_travel_layer_0 by default have the same ratio to the parent print speed as the travel speed has on other layers.

fieldOfView/Cura,senttech/Cura,hmflash/Cura,ynotstartups/Wanhao,totalretribution/Cura,senttech/Cura,Curahelper/Cura,hmflash/Cura,Curahelper/Cura,fieldOfView/Cura,totalretribution/Cura,ynotstartups/Wanhao