Add tileset loaded event

[lilleyse]

Adds a tileset loaded event that's broadcasted when the all tiles are loaded for the current view. This will be helpful for #261. Inspired by the implementation in Unreal: CesiumGS/cesium-unreal#906.

To test, add this code to _setup_subscriptions in main_window.py:

        tileset_loaded_event = carb.events.type_from_string("cesium.omniverse.TILESET_LOADED")
        self._subscriptions.append(
            bus.create_subscription_to_pop_by_type(
                tileset_loaded_event,
                lambda _e: self._logger.warn("Cesium tileset loaded {}".format(_e.payload["tilesetPath"])),
                name="cesium.omniverse.TILESET_LOADED",
            )
        )

You'll see "Cesium tileset loaded" messages printed in the console.

loaded.mp4

[weegeekps]

LGTM