Patch to trunk: merge common code in hourglass impl

[Adrian Robert]

Hello,

When I updated the hourglass implementation in the NS (Cocoa) port for  
the hourglass changes of 2008-03-19 I put some code that was  
duplicated in {x,mac,w32}fns.c into xdisp.c, leaving only  
{show,hide}_hourglass functions for the port to implement.  Other GUIs  
could use this common code as well, and here is a patch against trunk  
to do so.

There are three issues:

- I have tested only the X version.

- Under W32, atimers are not currently used, so the common code cannot  
be either.  If atimers will work under W32, then the patch can be  
improved to benefit W32 as well.

- Under Mac, the hourglass impl is under #ifdef MAC_OSX.  I replicate  
this constraint in xdisp.c as "#if !defined (MAC_OS) || defined  
(MAC_OSX)".  Hopefully this is correct.

(Patch attached)

hourglass_common.patch
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