Add table aliasse

Aliases is going to store different ISBNs for the same book, as well as
abbreviations like 'm5' for 'Lambacher Schweizer 5'.
Add alias functionality

Any ISBN/abbreviation entered into a DeleteDialog or InsertDialog will now be
looked up in the 'aliasse' table before being inserted or deleted from the
respective table, which allows a book to have more than one ISBN and an
abbreviation. Furthermore, the 'aliasse' table can be edited from within
Obsidian.
Fix minor issues

In FindDialog::getQuery(), the test for emptyness was changed from
»<any QString> != QString()« to »!<any QString>.isEmpty()«.