a nested Window automatically becomes transient for its parent

If you declare Window { Window {}} or Window { Item { Window {}}}
the inner window will automatically become transient for the
outer one.  The transient relationship must be set before the
inner window becomes visible though, so declaring visible: true
doesn't always work, depending on initialization order.  It's OK
if you assign visible (or call show()) later on when the user needs
to see the transient window for the first time.

Also added documentation.

Change-Id: I888c3b9da6d44d11516227c085bcc12a5ccb5e81
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>