Widget and viewer improvements

* Add `preRender` and `postRender` events to `CesiumWidget` and mirror them in `Viewer`.
* Make `Viewer` expose `CesiumWidget`'s `renderError` event instead of creating its own.
* Rename `CesiumWidget.onRenderLoopError` to `renderLoopError`.  `CHANGES.md` says we did this in b23, but we lied.
* Make `viewerCesiumInspectorMixin` use the `postRender` event instead of replacing the render loop entirely.