state callbacks of the modelviewer

[KevinRohn]

I added following callbacks:

  /// Invoked once when the model viewer is created.
  final VoidCallback onModelViewCreated;

  /// Invoked when the model viewer has finished loading the url.
  final VoidCallback onModelViewStarted;

  /// Invoked when the model viewer has finished loading the url.
  ///
  /// Please note: This function is invoked when the url has finished loading,
  /// but it doesn't represents the finished loading process of the model visibility.
  final VoidCallback onModelViewFinished;

  /// Invoked when the model viewer has loaded the model and the model is visibile.
  /// See: https://modelviewer.dev/docs/#entrydocs-loading-events-modelVisibility
  final VoidCallback onModelIsVisisble;

  /// Invoked when the model viewer has failed to load the resource.
  final ValueChanged<String> onModelViewError;