WelcomePage: Fix minimum size

The Welcome page should not impose a minimum size on Qt Creator.
We use a ScrollArea instead. This was a regression introduced by porting
to Qt Quick 2.

Task-number: QTCREATORBUG-10505
Change-Id: I5bbe4a18bdb96e91723987995a3632487bab536d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>