Set groundwork for better disabled reasons with range select

Something dumb that we're currently doing is expecting list items
to define an ID method which returns a string. We use that when copying
items to clipboard with ctrl+o and when getting a ref name for diffing.

This commit gets us a little deeper into that hole by explicitly requiring
list items to implement that method so that we can easily use the new
helper functions in list_controller_trait.go.

In future we need to just remove the whole ID thing entirely but I'm too
lazy to do that right now.

pkg/gui/controllers/stash_controller.go

@@ -24,6 +24,7 @@ func NewStashController(
 			c,
 			c.Contexts().Stash,
 			c.Contexts().Stash.GetSelected,
+			c.Contexts().Stash.GetSelectedItems,
 		),
 		c: c,
 	}