Web-based Search should support web-fetcher syntax

[koppor]

As user, I "just" want to search

• If I enter a DOI in the web search, the DOI should be searched (independent of the selected fetcher)
• When I enter a custom key/value pair, I want that key/value pair to be passed to the fetcher (and not be deleted)
• When I enter a syntax custom to the fetcher, I want that syntax be passed to the fetcher (and not be rejected). Example: Search in GVK: User may use Lucene syntax or GVK syntax.

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Implementation Idea:

1. Check for Id --> run id fetcher
2. Check for syntax of fetcher -> if match -> pass that
   • Is there a parser for the fetcher?
3. Check for lucene syntax --> if match -> pass that
   • Lucene might return that it cannot parse a field --> maybe raw needs to be sent
4. Show error

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Can we show the "mode" in the UI - like we did at the magnification glass at the search in JabRef 2.x?

• Valid query, just terms:
• Valid query, advanced search active, something found:
• Valid query, advanced search active, nothing found:
• Valid query, basic search active, something found:

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Currently:

• Invalid query:

This is currently sketched in the FetcherDelegate: https://github.com/koppor/jabref/pull/559/files#diff-6038155da69f4a45d14bde97e0718d663aa19d6e181eecbfe938b1c65d548cf8