DB: Added DBConnection class.
DB: Added QueryResult class.
TEST: Tested DBConnection and QueryResult classes.

Added a simple unit test for the DBConnection and the QueryResult
class.
VIEW: Added SELMainLibraryWidget class.
VIEW: Added SELUserLibraryWidget class
VIEW: Added SELUserLoansWidget class.
VIEW: Added SELUserAccountWidget class.
VIEW: Added SELMainWidget class.
VIEW: Added SELMainWindow class.
TEST: Added unit tests for view widgets.

These tests do not check functionality, just the user's interface
outer shell.