Hook for idle sync

[Antoine Levitt]

(I was sure I had posted this, but it doesn't appear to have been,
according to gmane - if it has, sorry)

I guess it's a pretty common use case to use postsynchook to display a
notification of some sort on new mail. This is overlooked by the idle
code, which doesn't call any hooks after getting new mail, thus losing
much of the point of idle for me. The idle code should call either
pre/post synchook, or a new hook.

What do you think?