wacky async GetProperty hack

[Havoc Pennington <hp redhat com>]

Hi,

Keith foolishly told me how to implement an asynchronous GetProperty
routine, so that's in metacity/src/async-getprop.c

It seems to be about 5 times faster to get several properties this way
than to get the same properties with XGetWindowProperty(), even on a
local X connection. For remote connections you'd expect even more
gain. It's hard to get an exact benchmark because my current
implementation adds an _XAsyncHandler for every GetProperty and is
thus O(n) in number of outstanding requests.

Probably a bigger win for metacity/libwnck than for GTK, and probably
not worth it even then, but still a fun experiment. It could be worth
it over a remote connection I suppose.

Havoc