compositor: Remove weston_output::move_signal

Since that signal is per output, it is necessary to track in which
output a view is in so that the signal is handled properly.

Instead, add a compositor wide output moved signal, that is handled by
the shell. The shell iterates over the layers it owns to move views
appropriately.
diff --git a/desktop-shell/shell.h b/desktop-shell/shell.h
index dbb2854..4d4f00a 100644
--- a/desktop-shell/shell.h
+++ b/desktop-shell/shell.h
@@ -184,6 +184,7 @@
 	enum animation_type focus_animation_type;
 
 	struct wl_listener output_create_listener;
+	struct wl_listener output_move_listener;
 	struct wl_list output_list;
 
 	char *client;