Doc: the Welcome screen was updated

Task-number: QTCREATORBUG-10446

Change-Id: I1bfda7119ebd541d4dbfa1b7a65c3155221b3c6d
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Doc: replace Qt GUI Application with Qt Widgets Application

Qt GUI application could also be a Qt Quick application.
Update screenshots.

Task-number: QTCREATORBUG-10263

Change-Id: I8c0dd34e1222c3b42fddaf3f562e402490ce1037
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>