Make "unlinked files" dialog a wizard

[koppor]

Unlinked files dialog is now a simple dialog that makes the user work step by step. select library, select directory, select file extensions to look for. Then search for files. Then select from the search results the files to import.

This could be improved to a proper assistant. ControlsFX offers a wizard class. controlsfx/controlsfx#1542 should be merged soon - this is a pre-condition.

Background

• Current dialog: https://docs.jabref.org/collect/findunlinkedfiles#using-the-wizard-search-for-unlinked-local-files
• Pattern "Assistent": http://www.usabilitypatterns.info/catalog/patterns/assistent.html

[Sikandar1310291]

assign me

[github-actions]

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