Add constructors for creating Preferences instances

The 2 constructors are useful when a new set needs to be created if there is
no matching ID. The constructor with no parameter is used when there is an
invalid last user file containing nothing (it should normally have 1 or 2
lines, representing the username and the user ID, respectively).
Implement method that loads the last user and creates a new user if none
Implement method that chooses random words and put them onto the JPanel

A new method that checks whether the given word will fit in the also given
width is implemented.