Support multiple outputs

Outputs are arranged in a horizontal layout in the order they are
created in by wlroots. Maximized xdg_shell views will span all outputs,
like the global fullscreen mode in sway.

Fixes #87

output.h

@@ -19,6 +19,8 @@ struct cg_output {
 	struct wl_listener destroy;
 	struct wl_listener damage_frame;
 	struct wl_listener damage_destroy;
+
+	struct wl_list link;
 };
 
 void handle_new_output(struct wl_listener *listener, void *data);