[Suggestion] .x model selectable options support

[hotdamndel]

This is a feature found in .x models and used by, for example, Railsim (a Japanese railway layout simulator). In Railsim example, this allows defining various model parts that can be enabled/disabled or swapped (for things like implementing train variety coming from small changes during production or car numbers) much more easily than conventionally via [object] and StateFunction in .animated. I assume this can be considerably less performance-costly as well. No particular examples here, but any Railsim model with a good functionality can demonstrate that.