3D Tiles - clean up underscored variables

3D Tiles files often use underscored private variables of other 3D Tiles files. One example is `Cesium3DTilesetTraversal` accessing `tileset._selectedTiles` and `tileset._requestedTiles`. Instead these should either be non-underscored private properties, functions, or part of a separate shared class.

Start by reading Cesium3DTilesetTraversal and the Cesium3DTileset usages of _selectedTiles and _requestedTiles. Trace how these values cross 3D Tiles class boundaries, then determine whether the access should use non-underscored private properties, functions, or a shared class. Done means the cross-file underscored accesses are removed without changing the traversal behavior.

Written by the indexing model from the issue text.