Create a loginHandler
FIX: Set the created DatabaseConnection to the instance

Otherwise it was created every time "getInstance()" was called.
Add a namedQuery to find Users by there Usernames
Add the login logic to the login handler

The LoginHandler requests the user with the corrisponding name from the
database and checks if the password is correct.
If everything is fine it moves on to the overview.
Otherwise it throws a LoginException.
Use the new login logic in the LoginPanel