Added: a 'Walkthrough: starteFooter' event when the user re-opens the...

Added: a 'Walkthrough: starteFooter' event when the user re-opens the walkthrough from the footer (instead of 'started', for the first time).

Added: a 'WalkthroughExited: $step' event when the user closes the walkthrough (unless it's the last step, in which case 'finished' is trigerred).

Fixed: the 'Walkthrough: finished' event was triggered twice.

Ref #468