Fix for #1442 - Find better names for ApiExplorerProperties

Taking the suggestion here to move these to a sub-object. This is future
proof in the event that we need to capture more data for ApiExplorer, and
reads better.
should not be possible for `action.ApiExplorer==null` or `controller.ApiExplorer==null`.  use `.' here and at line 305, as in `DefaultControllerModelBuilder`