The SelectableMixin adds behavior to a control which can be selected. It adds the following behavior:
- Raises an IsSelectedChangedEvent when the value if the IsSelected property changes.
- Adds a 'selected' class to selected controls.
- Requests that the control is scrolled into view when focused.
Mixins apply themselves to classes and not instances, and as such should be created in a static constructor.
- static Initializes a new instance of the SelectableMixin class.