List of clients

Hi!

Not all clients seem to be available through `client.get()`. What the
criteria for a client to be in this list? For example, I am trying to
get x2x in the list of clients but it does not seem available, even if
it does create a small window. I am able to spot it with `xwininfo` (but
not with `xlsclients`):

     0x3200003 "x2x localhost:10.0": ()  2x1076+3838+2  +3838+2
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