Multiple code improvements

The total line number and character count has been reduced by multiple
enhancements. These include:
- Removement of enterlistener class
- Added meathod newNumbers()

I moved the meathod somethingHappened() to the MathTest class after
implementing the ActionListener's and KeyListener's meathods in the
same class. That removed the need for the extra class.

The newNumbers() meathod generates new random numbers, depending on the
difficulty level setting, and asks the main question.

Another main issue has been fixed: When starting a new game after
playing one, the player has to answer the first question from the
Middle School difficulty level. Now the user can choose what difficulty
level before starting a new game.