Pass focus events down

[andrewbaxter]

I can add this, although I'm not sure how far/when I'll get that far in my current project.

Basically it would be useful for the program to know if the terminal gains/loses focus for things like showing notifications. Those are passed in to the line but not exposed to the user currently.

I think it would be a straightforward addition, just adding new focus events to the current readline event enum.