Refactor API

APIs to consider removing/simplifying:
 - MapToolbar `addButton()` and `addModeButton()` now that we are close to getting an [addToolButton()](https://github.com/TerriaJS/terriajs/blob/main/lib/ViewModels/MapNavigation/MapToolbar.ts)
 - more?

Remember to fix the DTV plugins after the change.