Add classmethod label() to Operator (and maybe icon() too!)

We have the very handy @classmethod description() in Operators, and I'd like to suggest a @classmethod label() for the same reason.

Sometimes when making panels you want the same operator to perform different actions based on the current context. It's better than having the panel make the decision what to label the operator button.

If we could get the same thing for @classmethod icon() that would be lovely.