Fix edits getting lost in Biblatex mode

This fixes a bug where edits made in a field on a
CompressedEntryEditorTab wouldn't get stored when the field editor lost
focus.
The bug was caused by EntryEditorTabFocusListener accessing the unused inherited parent field
of CompressedEntryEditorTab instances instead of the latter class's own (private) parent field.
Instead of doing just a quick fix, I decided to eliminate the excessive code and data duplication
in the two classes altogether and consolidate them into one class (with an additional flag to the constructor),
since I don't think an extra class is warranted here (and easily and sensibly implementable by derivation).