Use Flow for extruder boxes instead of GridLayout
Use Flow for extruder boxes instead of GridLayout

Flow makes things a lot more simple with the double-width item at the bottom.

Contributes to issue CURA-3161.

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