Realtime CSG messes with non-CSG prefab selection

[Candescence]

Even when you don't have any CSG elements in a scene, Realtime CSG will cause scene view selection to automatically select a prefab's child, which can be extremely undesirable behaviour in many circumstances, particularly when placing prefab objects in a scene for level design.

Discovered in Unity 2018.1.0f2.

[LogicalError]

I'll take a look

[LogicalError]

somewhat related to #250

[loosegrid]

CSGModelExported has the [SelectionBase] attribute, which causes this issue in prefabs with exported models. There may be other situations where rcsg messes with non csg selection as well.

Commenting out the attribute causes the prefab root to be selected normally.