view/xwayland: avoid focusing views that don't want focus

XWayland views can self-declare that they don't want keyboard focus via
the ICCCM WM_HINTS property. Most of the logic is already in place to
avoid giving focus to such views (e.g. taskbars).

Add a couple of missing pieces to make this work:

- Hook up view_isfocusable() to look at WM_HINTS for XWayland views
- Adjust desktop_focus_topmost_mapped_view() to skip unfocusable views

src/seat.c

@@ -496,7 +496,7 @@ seat_set_focus_layer(struct seat *seat, struct wlr_layer_surface_v1 *layer)
 {
 	if (!layer) {
 		seat->focused_layer = NULL;
-		desktop_focus_topmost_mapped_view(seat->server);
+		desktop_focus_topmost_view(seat->server);
 		return;
 	}
 	seat_focus(seat, layer->surface);