Improve CSD logic

This does the following:

* Removes the xdg-decoration surface_commit listener. I was under the
impression the client could ignore the server's preference and set
whatever decoration they like using this protocol, but I don't think
that's right.
* Adds a listener for the xdg-decoration request_mode signal. The
protocol states that the server should respond to this with its
preference. We'll always respond with SSD here.
* Makes it so tiled views which use CSD will still have sway decorations
rendered. To do this, using_csd had to be added back to the view struct,
and the border is changed when floating or unfloating a view.

include/sway/xdg_decoration.h

@@ -10,7 +10,7 @@ struct sway_xdg_decoration {
 	struct sway_view *view;
 
 	struct wl_listener destroy;
-	struct wl_listener surface_commit;
+	struct wl_listener request_mode;
 };
 
 struct sway_xdg_decoration *xdg_decoration_from_surface(