cleanup StackedTileLoader

* The StackedTileLoader doesn't calculate the visibility of layers on the fly any more.
Instead, it gets a list of layers that it should handle.
Visibility changes of layers are signalled to the TextureLayer which re-calculates
the list of visible layers and passes the new list to the StackedTileLoader.
See TextureLayer::Private::updateTextureLayers().

* The TileLoader used to hold all GeoSceneTextures from all map themes and continues
to do so. However, it now manages the set of GeoSceneTextures itself rather than
relying on the StackedTileLoader.
See TileLoader::updateTextureLayers().