Expose map bounds

[xqwzts]

Provides a way for clients to get the current map bounds.

Fixes #87

• Calculates the map bounds on move.
• Caches the value in MapState and exposes a bounds getter.
• Adds bounds property to MapPosition, and provides it upon triggering the onPositionChanged callback.
• Adds bounds getter to MapController/MapControllerImpl.
• Updates the MapController example with a button that shows the current bounds in a SnackBar.

[johnpryan]

Thanks for the PR - I'll take a look as soon as I can

[johnpryan]

Thanks for the contribution!