text: Assign text_model to a wl_seat

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.
diff --git a/src/compositor.c b/src/compositor.c
index bdf9459..30a1f4f 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -2625,6 +2625,7 @@
 		      &seat->new_drag_icon_listener);
 
 	clipboard_create(seat);
+	input_method_create(ec, seat);
 }
 
 WL_EXPORT void
@@ -3173,7 +3174,6 @@
 
 	screenshooter_create(ec);
 	text_cursor_position_notifier_create(ec);
-	input_method_create(ec);
 
 	wl_data_device_manager_init(ec->wl_display);