Put in place the basic infrastructure to allow supporting multiple
desktop windows.

diff --git a/server/user.h b/server/user.h
index 44d8f38..5bf3735 100644
--- a/server/user.h
+++ b/server/user.h
@@ -38,6 +38,8 @@
     USER_HOOK
 };
 
+#define DESKTOP_ATOM  ((atom_t)32769)
+
 /* user handles functions */
 
 extern user_handle_t alloc_user_handle( void *ptr, enum user_object type );