Appbar improvements

- Make back button work - it's shown when `onBackButtonClick` property is given (same as nav button).
- Add optional `onTitleClick` property.
- In `checkExpanded()`, ensure the component is still mounted before trying to do state changes.

Note that there's also unused `onMenuButtonCLick` property (also note the typo in "click"), I didn't do anything with it since it's unclear to me how it's different from nav button. Should it be removed?