Wpf Example Change TabControl to ItemsSource Take #3

Minimal set of changes to convert the WPF example into a ItemsSource based TabControl
- Uses DataTemplates for ViewModel->View mapping
- Imported TabControlEx - required so TabItems don't re-rendered upon changing of tabs
- Included an initial set of key bindings for adding and closing tabs

Removed "code noise" changes from previous PR, keeping this as concise as possible. A few smaller commits should make the changes easier to follow.