GDK W32: Don't use SetWindowLong() to set/unset WS_EX_TOPMOST

GDK W32: Don't use SetWindowLong() to set/unset WS_EX_TOPMOST

While searching for the cause of bug 746745 it was discovered that one could
not set WS_EX_TOPMOST extended window style with SetWindowLong(),
but must use SetWindowPos() for that purpose.

This was never a problem most likely because it is highly unlikely for windows
to acquire/lose WS_EX_TOPMOST after they are created, by means other
than SetWindowPos() (which GTK does use to raise/lower windows and
set/remove keep_above), and because trying to set/unset WS_EX_TOPMOST with
SetWindowLong() results in WS_EX_TOPMOST merely not being set/unset (that is,
other styles are still set/unset within the same call and no error is
signalled).

https://bugzilla.gnome.org/show_bug.cgi?id=758483