compatibility with emacs22

Emacs22 lacks apply-partially and mouse-event-p, so define them if emacs
version is less than 23.  With this change, I was able to begin using
notmuch in emacs22.

The definitions of apply-partially and mouse-event-p are copied from
the emacs 23 distribution, (which is distributed under the GPLv3+ just
as notmuch).