Added new tab with ListBox to sampleapp.
First attempt to add support for ListBox. Not working because ListBoxItems are not created for non-visible items.
Shorten list of muppets in ListBox so that no items are lazy-created. Makes test pass and I can focus on solving the lazy problem.
Let AllItems on ComboBox return ComboBoxItem. Must be opened for that to work. Replace IContentControl with finding inside visual tree.
Experimenting with simpler By for finding ComboBoxItems.