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/workspaces.c

@@ -289,7 +289,7 @@ workspaces_switch_to(struct workspace *target, bool update_focus)
 	if (update_focus) {
 		struct view *view = server->focused_view;
 		if (!view || !view_is_always_on_top(view)) {
-			desktop_focus_topmost_mapped_view(server);
+			desktop_focus_topmost_view(server);
 		}
 	}