SLR UI for queries

[koppor]

For each query:

• Title
• (List of databases)
  • database: database-specific query - if not defined, show in gray how the lucene query is transformed to database-specific query. If users clicks into the field, this text is the content of the text field. (In this way, the user can start with a "good enough" default)

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• Tree-based structure

[koppor]

Plus: Adapt YAML (See https://github.com/koppor/jabref/pull/559/files#diff-8686084b4cb107dc34f1352c8d8ced6214d03abfb2a28900fad6f6954ab276fa)

• "refined query" -> "library-custom"
• read/write of YAML should work
• pass to org.jabref.logic.crawler.StudyFetcher#StudyFetcher is unmodified (thus, the custom queries are not sent to the code) -- in this way, we can merge the PR fast :)