Create CheckableListItem component
This is a BasicListItem with a Checkbox assigned to its
property. When an app adopts it, the app is free to use the list item's
checked: property to actually control whether the list item is checked,
which is something you currently cannot do with BasicListItem itself (it
used checked: synonymously with "selected").
This is an alternative to !89 (closed), which ran into the above issue.
Here's how it looks when used for Discover's Addons view:
(the incorrect separator paddings between items are an unrelated bug that I will fix separately)