Fixed a number of bugs in the handling of window parent and owner and
added a regression test (based on the work of Bill Medland).

diff --git a/server/protocol.def b/server/protocol.def
index fd526e6..6e69e93 100644
--- a/server/protocol.def
+++ b/server/protocol.def
@@ -1751,6 +1751,7 @@
     user_handle_t  owner;       /* new owner */
 @REPLY
     user_handle_t  full_owner;  /* full handle of new owner */
+    user_handle_t  prev_owner;  /* full handle of previous owner */
 @END