Create CheckableListItem component

This is a BasicListItem with a Checkbox assigned to its leading: 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)