Add CSD to border modes

This replaces view.using_csd with a new border mode: B_CSD. This also
removes sway_xdg_shell{_v6}_view.deco_mode and
view->has_client_side_decorations as we can now get these from the
border.

You can use `border toggle` to cycle through the modes including CSD, or
use `border csd` to set it directly. The client must support the
xdg-decoration protocol, and the only client I know of that does is the
example in wlroots.

If the client switches from SSD to CSD without us expecting it (via the
server-decoration protocol), we stash the previous border type into
view.saved_border so we can restore it if the client returns to SSD. I
haven't found a way to test this though.

sway/ipc-json.c

@@ -216,6 +216,8 @@ static const char *describe_container_border(enum sway_container_border border)
 		return "pixel";
 	case B_NORMAL:
 		return "normal";
+	case B_CSD:
+		return "csd";
 	}
 	return "unknown";
 }