No way of determining whether an Item is visible or not

There is currently no reliable way of determining whether a given QML element
is really visible or not. I.e. given a reference to a QML element, it may have a
non-zero opacity value yet be invisible as a result of a parent element having
zero value opacity.

Either a new API for querying the Item's opacity including any inherited effect is
needed, or a new isVisible property. Note that the latter would need to also take
into account any opaque element(s) obscuring the item.