Make sure stacks will have non-empty definitionChanges in project loading
Make sure stacks will have non-empty definitionChanges in project loading

CURA-4107

In old Cura, there is no definitionChanges container, so loading a
project file from an old version will result in stacks with empty
defnitionChanges. This will cause an unnecessary auto-slice problem when
we switch between the extruder tabs for the first time. This action will
trigger a piece code in MachineAction which creates a definitionChanges
container for the active stack if it is empty. This eventually triggers
a propertyChanged signal and results in an unncessary auto-slicing.

Curahelper/Cura,Curahelper/Cura