BibEdit: necessary auto-complete corrections

• The constants used to communicate the bibEdit state are introduced in order to avoid the usage of bare numeric values.

• AJAX request associated with a value autocompletion has been separated. (Previously, the requests of this type, were served by the same handler as regular record updates, which was not fitting logically very well. In addition, after extending the former handler with code realising functionalities characteristic to the record updates, such as treatment of Undo/Redo handlers or of attached Holding Pen change information, the autocomplete code was not functional any more.