Option to nullify sub-buildings

[algernon-A]

Some workshop assets, like this one and one of these, contain sub-buildings that are set to e.g. monument AI. Should add an option (on by default, with option in settings to be overridden, including by author settings) to convert these to dummy AI instead.

[shulima]

I'm not sure if it's the same issue, but this parking garage also appears to contain sub-buildings that retain attractiveness even after converting to a dummy asset. (Just in case you wanted another sample to test on.)