Getting notified on focus events.

[martin.blais@gmail.com]

Hi

New gig, new environment, and I inherited two glorious 30-inch LCD
monitors and a 24-in sidekick.  Nice setup one would think, but my
emacs windows are so unscrupulously large now that it takes me about 10
seconds to find where my cursor is, even with blinking on.  To
alleviate the problem, I wrote an Emacs package that will briefly flash
an overlay around the cursor when I switch frames.
http://furius.ca/pubcode/pub/conf/common/elisp/blais/highwin.el

While this is working well when switching between frames, I also need
to flash my cursor when I switch between window manager windows (e.g.
non-Emacs windows, the X-windows of other applications).  That is, I
need to be notified for enter-focus and leave-focus X events, somehow.

I tried adding an advice on handle-switch-frame and make-frame-visible
and a few other functions, and I cannot get it to work.  There are no
hooks I could find.

Any idea?  Can this be done?