xwayland: handle unset _NET_WM_NAME

The spec says [1]:

> If set, the Window Manager should use this in preference to WM_NAME.

However we overwrite WM_NAME with NULL when _NET_WM_NAME is unset.
Fix this by storing both WM_NAME and _NET_WM_NAME, so that we
handle properly all combinations of events (e.g. a client setting
both and later clearing one).

[1]: https://specifications.freedesktop.org/wm-spec/1.3/ar01s05.html#id-1.6.2

include/wlr/xwayland/xwayland.h

@@ -232,6 +232,8 @@ struct wlr_xwayland_surface {
 	void *data;
 
 	struct {
+		char *wm_name, *net_wm_name;
+
 		struct wl_listener surface_commit;
 		struct wl_listener surface_map;
 		struct wl_listener surface_unmap;