Proposal: Enable ItemsView selection via binding

# Proposal: Enable ItemsView selection via binding
Currently the `ItemsView` only enables selection by index, and it's achieved by calling `itemsView.Select(index)`. Please convert the currently read-only `SelectedItem` and `SelectedItems` properties to dependency properties so we can bind to them efficiently and in a loosely coupled way from the view-model.

## Summary
The only way to invoke a selection of items in the `ItemsView` is by calling `itemsView.Select(index)` or `itemsView.SelectAll()`. There should be a better more MVVM-friendly way to achieve that.

## Rationale

* ItemsView is not MVVM-ready
* ItemsView should enable selection via binding

## Scope

| Capability | Priority |
| :---------- | :------- |
| `ItemsView.SelectedItem` should be a read-write dependency property | Must |
| `ItemsView.SelectedItems` should either be a read-write dependency property or become a modifiable `ObservableCollection` | Must |

Start by locating the ItemsView implementation and the existing SelectedItem, SelectedItems, Select(index), and SelectAll APIs. Inspect how selection state is currently stored and exposed. Done means the requested selection properties support binding as described in the scope, with selection behavior covered by the relevant tests.

Written by the indexing model from the issue text.