ListView doesn't update properly

Sometimes we get to crashes if the objects item is passed back to C++.

Something like would crash as forwardTo will have stored dangling pointers when the ListView decides to clear delegates in some occasions.

ListView {
    header: TextField { id: filter }
    model: SomeModel { filter: filter.text }
    Keys.forwardTo: [currentItem]
}

Related bug reports in our bugzilla:
https://bugs.kde.org/show_bug.cgi?id=381814
https://bugs.kde.org/show_bug.cgi?id=389168
(and duplicates)

It always refers to this forwardTo sending events to objects that don't exist anymore.
Patch follows.