a532cfb475 added `isCollection()`, `isSearch()`, and `isItem()` methods
to data objects to handle collections and searches in the trash, with
`isItem()` checking whether `._ObjectType` was `Item`. That left out
feed items (`._ObjectType` == `FeedItem`), and when c384fef867 made
`getSelectedItems()` return only items, it used `isItem()`, so feed
items were excluded, which broke feed-item toggling between read and
unread [1] and possibly some other things.
The simple fix would be to make `isItem` match feed items as well (which
could potentially fix other bugs related to feed items), but there was
actually no need to add new methods (which can get confused with
`CollectionTreeRow` methods) when we can just check the object type with
`obj instanceof Zotero.Item`, which gets the benefit of inheritance and
matches `Zotero.FeedItem` instances as well.
[1] https://forums.zotero.org/discussion/115571/cannot-change-the-status-of-title-in-subscribtion
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