text: Have only one text_model_factory

Have only one text_model_factory instead of one per seat.

This commit also introduces destruction of an input method when the
corresponding seat is removed.

Signed-off-by: Jan Arne Petersen <jpetersen@openismus.com>
diff --git a/src/compositor.h b/src/compositor.h
index 0d1699f..6716bdc 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -747,6 +747,9 @@
 text_cursor_position_notifier_create(struct weston_compositor *ec);
 
 void
+text_model_factory_create(struct weston_compositor *ec);
+
+void
 input_method_create(struct weston_compositor *ec,
 		    struct weston_seat *seat);