Tweak the back and forward buttons

To have a more "discoverable" UX, and cleaner UI (requiring private
QML API for now).

Change-Id: Ic275287d429149a1c353a1cbdc5b7f9b19e57410
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>