Mixin which maps a click (actually, a mousedown) to a selection.
This simple mixin is useful in list box-like elements, where a click on a list item implicitly selects it.
This mixin expects the component to provide an items property. You can
provide that property yourself, or use
ContentItemsMixin. This mixin also expects the
component to define a selectedIndex property. You can provide that
yourself, or use SingleSelectionMixin.
Kind: global class