Hello.

I'd like to contribute the attached patch, which adds hooks into gain and
loss of focus events.

This allows saving all buffers on loss of focus.  (Alt-tabbing between
an editor and browser is a popular workflow for web programmers.)

It also allows reversion of buffers on regain of focus, convenient if
one is switching between Emacs and another editor.

People will probably find other handy uses for these hooks.

Best,
Brian Jenkins