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

include/view.h

@@ -69,6 +69,8 @@ struct view_impl {
 	struct view *(*get_root)(struct view *self);
 	void (*append_children)(struct view *self, struct wl_array *children);
 	struct view_size_hints (*get_size_hints)(struct view *self);
+	/* if not implemented, view is assumed to want focus */
+	bool (*wants_focus)(struct view *self);
 };
 
 struct view {