TryClientEvents already did this; this commit just moves the assignment
 one level down so that no event source has to worry about sequence
 numbers.

 ...No event source, that is, except XKB, which inexplicably calls
 WriteToClient directly for several events.
Signed-off-by: Jamey Sharp <jamey@minilop.net>
Reviewed-by: Julien Cristau <jcristau@debian.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>
 Backport to nx-libs: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>