"Blank 3D Tiles Tileset" add button should account for selected georeference

[j9liu]

If multiple CesiumGeoreference actors are in the scene, every "Blank 3D Tiles Tileset" added from the Quick Add panel will go under the first georeference in the scene. This happens even when a different georeference is selected. It would be a nice quality-of-life improvement if this accounted for the CesiumGeoreference that was selected.