This patch replaces the old Qt logo with the new, flatter one.

The PNGs were optimized via: optipng  -o7 -strip "all"

Task-number: QTBUG-41685
Change-Id: I51983a45144373bf7aee31a32990ecbb2582f488
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>