Show more information on the fields of a table in the DB Structure view

In the Database Structure tab of the main window and the DB Schema
pane, show more information in the field nodes of the tree view. This
commit puts the field definition in the Schema column which has been
unsused until now. It also changes the icon of the field row when the
field is a primary key. This hopefully makes the Schema view a bit more
useful, especially in the pane.