Add indicator for existing items

[davidjaggi]

Add an indicator to the search bar to highlight if an item is already imported.
This prevents duplicate entries of items. Similar to https://github.com/daeh/zotero-markdb-connect.

[vyleung]

would you prefer reducing the opacity of the item, adding a check mark (or another icon) next to the item name, or something else?

[davidjaggi]

I guess a checkmark would be nice. A change in opacity points to a problem with Zotero items, so out of this context, it could mislead users who use it for the first time. It should be easily visible but not distracting.