Add NotifyTargetSet

[Jamiras]

Replaces the common pattern of:

    NotifyTargetSet vNotifyTargets(m_vNotifyTargets);
    for (NotifyTarget* target : vNotifyTargets)
    {
        Expects(target != nullptr);
        target->OnDataModelBoolValueChanged(args);
    }

with

    if (m_vNotifyTargets.LockIfNotEmpty())
    {
        for (auto& target : m_vNotifyTargets.Targets())
            target.OnDataModelBoolValueChanged(args);

        m_vNotifyTargets.Unlock();
    }

Which provides three small performance boosts:

1. Avoids the copy/iterate if the list is empty
2. Avoids the copy entirely unless the list is modified while it is locked
3. Avoids the null check inside the for loop

None of these are very expensive by themselves, but ever PropertyChanged event uses this construct to propogate the event to the handlers. And those may chain into other handlers and additional PropertyChanged events.

This shaves roughly a dozen microseconds off each frame.