Reach out to the contentItem inside of the ListView to find the width

Reach out to the contentItem inside of the ListView to find the width

Turns out that the ListView embedded in the Menu (reachable via
contentItem) would create a binding loop if we're using its
childrenRect. But really we're interested in the total width for the
instantiated delegate *inside* the ListView. That's why we go one level
deeper and get the childrenRect of the inner contentItem instead.

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>