wizard: Do a more thorough cleanup

Clean-up all state in cleanup() method.

https://bugzilla.gnome.org/show_bug.cgi?id=726967
wizard: Complete cleanup on cancellation

Cleanup all state on cancellation of wizard.

https://bugzilla.gnome.org/show_bug.cgi?id=726967
wizard: Complete cleanup on arriving to source page

Do a complete cleanup of wizard state when arriving to source page and
do so in both forward and backward directions. This, combined with
'wizard: Do a more thorough cleanup' commit, fixes the issue of user
ending up in the wrong wizard pages and getting stuck on preparation
page.

https://bugzilla.gnome.org/show_bug.cgi?id=726967