Added useEffect for back handling
Added useEffect for back handling

It's tricky because can't add/remove back handler every render (the default). If did that then back handler for scene 2 can be re-added after back handler for scene 3 if React chooses to re-render scene 2. But the handle back needs the latest value of the page so doesn't work if just stop useEffedct from re-running.
Used technique from Abramov's overreacted blog https://overreacted.io/making-setinterval-declarative-with-react-hooks/

grahammendick/navigation,grahammendick/navigation,grahammendick/navigation,grahammendick/navigation,grahammendick/navigation,grahammendick/navigation,grahammendick/navigation