Changed structure of commands

Each command has its own class, and somethingHappened() is now shorter.
Adding new commands is easier.
New debug screen that shows live values of variables, and removed static from lots of variables.

When I was a beginner programmer, I didn’t quite understand the idea of
static variables and methods. I was frustrated with the action listener
I used to use because it wanted my fields static, so I made everything
static in the program. Then, with the use of MathOperator, I made
everything static again.

Now I quite understand the concept of static, and I realize that my
program could break in a strange way if something were to go wrong,
like another MathQuizGame object initialized. I will be converting
almost all fields that don’t need to be static to non static by update
1.6.
Update private.xml