Blender Git Loki

Doversion: merge collection engine properties

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).