As an editor I want to query for values that link to Items in order to query for basic important data.

Problem:
We currently don't allow selecting Items in the value part of the query. A lot of basic and important queries ("instance of:country" etc) require it however. We need to support Item values.

Example:

• Instance of: country
• gender: female

Screenshots/mockups:
TODO

BDD
GIVEN a visual query
WHEN selecting a Property of datatype Item
THEN I can select Items in the value field of the query

Acceptance criteria:

• correct SPARQL code is generated
• type-ahead suggestions are available similar to current input on Wikidata (including input of Q-ID instead of label)
• only existing Items are suggested and can be selected

Notes:

• We use the lookup component for the value input