Wizard authors can now provide a persistence key to values where that
makes sense.
For now, we make use of it for the build system type in all wizards and
the minimum Qt version as well as the virtual keyboard settings in the
QtQuick wizards.
Fixes: QTCREATORBUG-16657
Change-Id: I179930665bd163b1cf198467ab8b43bc72f8ec4a
Reviewed-by: Eike Ziller <eike.ziller@qt.io>