Add case expression class for simple SQL case statements

References #4202

Usage is:
```
$query->newExpr()->addCase($expression, $valueIfTrue, $valueIfFalse)
```

`$expression` must be a QueryExpression instance.
I merely pointed out that I am seeing this the first time now - and that I think we should keep it consistent. Either way we see fit. I don't really have a preference, but so far we didn't space AFAIK.