This is a more complex approach, which makes me really want to use
IDProperty instead (assuming we handle their merging nicely).
In fact I would expect this to happen in readfile.c, not during
doversion, but I can revert this later.
For now this allow for demo files saved with 2.8 to keep working even
after we add/remove engine settings properties.
The values are merged. There is no purge though, so
old CollectionEngineSettings and CollectionEngineProperty will live
forever (for the time being).
e3dd2b1591