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.
We now have a (format) version and a setting version. Ideally we'd like major-minor version numbers in our profiles. However, introducing major-minor version numbers requires substantial changes to the version upgrade manager to compare version numbers, find a path towards the current version, or even keeping track of the current version. Therefore we just collapse the two version numbers into one: Multiply the major version number by a million and you'll never exceed it in the minor versioning. The only problem is that we now have to update the versioning for all of our three upgrade plug-ins, because they all need to know locally how to find the version number of their file types (because the upgrade manager has no knowledge of the file types) and they have no access to each other because a plug-in may be disabled.
Contributes to issue CURA-3427.
