Add a wl_seat argument to the activate and deactivate requests of
text_method.
On activation a text_model gets assigned to the input_method of the
wl_seat specified in the activate request.
This is necessary because all clients need a way to create
text_models, but only one client at a time can be bound to
the input_method global (else we don't know to whom we are
supposed to send events).
Jan Arne Petersen
Philipp Brüschweiler
Kristian Høgsberg