All of these patches add field to Gpm_Event, which is the quantum
of communication with gpm clients. Change Gpm_Event changes
the communication protocol, and thus is an incompatible change.

If the protocol has to be changed, we need to create a new channel
and still keep the old protocol. Also, the new protocol must be as
good and extensible as possible, to avoid any similar incompatible
change in the future.