CURA-4188
Custom FDM printers in 2.5 don't have multi-extrusion support but they
do since 2.6. Upgrading from 2.5 to 2.6 will not create the missing
extruder stacks for the old custom FDM printers, which causes a crash.
This fix makes sure that all custom FDM printers will have 8 extruder
stacks during the upgrade so that Cura can still start normally and
those printers will still be usable.
CURA-3975
- Set Preferences setting_version in CuraApplication so Preferences can
get upgraded correctly
- Fix upgrade script for 2.5 to 2.6
- Fix upgrade script for 2.6 to 2.7 which relies on the upgrade of 2.5
to 2.6