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.c b/src/compositor.c
index 77583b9..8cb8a3f 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -2769,6 +2769,7 @@
 
 	screenshooter_create(ec);
 	text_cursor_position_notifier_create(ec);
+	text_model_factory_create(ec);
 
 	wl_data_device_manager_init(ec->wl_display);