Integrity checker should link to entries via reference

[stefan-kolb]

Follow up form #1779

Linking to Bibentries inside the integrity checker should be done via a reference to the entry not the key.
We should switch it to BibEntry, i.e. just keep a reference to the entry and the use/create a method highlight(BibEntry)

[stefan-kolb]

• Bibtexkey is not focused correctly
• Focus by Id rather than Bibtexkey
• check for empty bibtexkey is working and correctly referencing the entry by ID