BibEdit: new undo/redo facility

• The possibility of undoing and redoing changes that have been made in the record editor has been added. The change history is remembered as long as the record cache exists.
  
  This functionality covers all the modifications of the internal structure of the record. Also operations like Copy/Paste and approval of Holding Pen changes. Some operations such as record removal or removal of the entire Holding Pen changeset are not undoable.

• This commit contains also several bugfixes to problems with creating new records and empty record history. (fixes: #42)