This is a pain:

 - The GtkWindodw mnemonic code needs to be able to handle sub-widget
   cycle positions, or we have to register a dummy widget per
   memnonic (ugh)

 - We have to be able to get positions of all the mnemonics across
   the plug/socket boundary. This is going to be a pain to maintain
   so it's probably best to do it at conflict resolution time,
   but that means a round-trip/synchronous message with attendant
   evils. (Though we should be able to handle it asynchronously
   within GTk+)