bug#15029: 24.3; feature-request: add window-focus-lost-hook

[Stefan Monnier]

>     We are talking about when the input focus enters and leaves Emacs in a
>     GUI environment.
> It's the same kind of issue.  I don't think Emacs should run any Lisp
> code when that happens -- it would make things hard to predict.

Following that same reasoning, we shouldn't have jit-lock.
And post-command-hook.  And mouse-autoselect-window.  And timers.  Etc...
These all make debugging at times difficult.

Yes, these things should be used with care, but that doesn't mean we
shouldn't provide them, because they can be very useful.


        Stefan