We should decouple focus and frame switching

[Daniel Colascione <dancol@dancol.org>]

Focus-in and focus-out don't do a good job right now of reflecting 
actual window system focus state, and that's in part due to the core 
considering frame switch evidence of a focus change and running the 
focus-in hooks in response. More than one frame can have window system 
focus at a time when we have multiple terminals, and even on a single 
terminal, internal frame switching for processing mouse wheel events and 
such shouldn't change the reported window system focus state.

How about this? We'll add a frame-focused-p function that returns nil, 
t, or 'unknown, and wire up focus-in-hook and focus-out-hook to run only 
when we actually observe a window system focus change, letting the frame 
switch stuff run completely independently. This way, we'll be able to 
write lisp (as I've been trying to do) that reflects the real window 
system focus.

Sound good?