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/view.c

@@ -118,6 +118,9 @@ view_isfocusable(struct view *view)
 	if (!view->surface) {
 		return false;
 	}
+	if (view->impl->wants_focus && !view->impl->wants_focus(view)) {
+		return false;
+	}
 	return (view->mapped || view->minimized);
 }