Attachment column icon not updated

[tnajdek]

In many scenarios, the column is not correctly updated. This includes:

• When the primary attachment is moved to/from the trash (this actually works: Double-clicking item opens attachment after deleting it #377)
• When the primary attachment is moved away from or back onto the item (not supported yet: It's impossible to attach existing standalone attachment to an item #454)
• When the primary attachment is replaced with one of a different type.

The Web Library currently does not track the primary attachment, instead relying on the value provided by the API, which only updates if the parent item is re-fetched. This does not happen for any child item change.

Since we know the key of the "best" attachment, we should keep track of it and update the icon as the primary attachment changes.