Purge libreis and properties, add sequence to START_EMULATING
Get rid of libreis since what it does is done by the portal out-of-band anyway. This means the wire communication gets a lot simpler since we no longer need to support two clients with (potentially) different versions on the same connection.
Then purge the properties too - the few things they were used for are/can be provided by the portal out-of-band.
Finally, add a sequence number to START_EMULATING - this will need an update in the docs to the InputCapture portal PR which currently specifies a property to be set to the InputCapture::Activated signal's activation_id. But I think it should work to just set this activation id as a sequence number in start emulating. This effectively forces an IC implementation to use stop emulating but... I can't think of a case where this isn't the sensible thing to do anyway.