Handling ambiguous ComboItem labels

Steps to Reproduce

run the attached example
select the item AA/AhhAhh-02

Actual Result

the comboValue1 in the VM is set to AA/AhhAhh-01

Expected Result

the comboValue1 in the VM should be AA/AhhAhh-02 (the same object that was selected)

Debug Info

when saving the binding selectedItem="@bind(vm.comboValue1)" the
ComboboxSelectedItemConverter executes during "onChange" - at that moment only the label "AA" is available in the event so the exact match cannot be retrieved

Root Cause

a combination of #2966241 (triggering the @save during onChange)
and ZK-1492 (retrieving the synchronizing the selectedItem by it's label)

Workaround

1. only use unique Comboitem labels
2. if ambiguous labels are necessary don't bind selectedItem =>it is save to retrieve the selected Model object from ListModel.getSelection()