Menu: Estimate item height from first visible item

The first item could be hidden, or not a normal item,
thus returning an unproper height. The consequence in
the reported issue being that the popup's maximum height
would be that of a scroll indicator.

Task-number: QTBUG-37445
Change-Id: Ib06d25643cd11224d5eab0452dadd0f6d4119fb9
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>