VIEW: Added get/set user id on SELMainWidget class.
BUG: Fixed memory leak on singleton DBConnection object.
CONTROLLER: Added SELController class.
SEL: Reestructured log in functionality.
MODEL: Added ItemModel class.
SEL: Started adding functionality to Main Library view.

Main Library view can now load a page of items from the database,
and obtain Book data when a book item is selected.