ui: add next and prior keysyms

Page-up and Page-down were renamed.  Add the names to the keysym list
so we can parse both old and new names.  The keypad versions are already
present in the vnc map.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170726152918.11995-2-kraxel@redhat.com