Fix string-type settings by making them specify default_value
Fix string-type settings by making them specify default_value

None of these settings get a 'value' property defined in fdmprinter, so they can safely use 'default_value' to specify their setting value.

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