Refactor submenu creation logic

src/activities/MenuListActivity.h

@@ -70,12 +70,18 @@ struct Rect;
 class MenuListActivity : public Activity {
  protected:
   std::vector<SettingInfo> menuItems;
+  std::vector<SettingInfo::SubmenuData> submenuData;
   int selectedIndex = 0;
   ButtonNavigator buttonNavigator;
+  bool submenusPrepared = false;
 
   // Call after building/rebuilding menuItems to wire up the selectable predicate.
   void initMenuList();
 
+  // Process SettingInfo items marked with withSubmenu() into submenu placeholders.
+  void prepareSubmenus();
+  void openSubmenu(const SettingInfo& submenuEntry);
+
   // Handle up/down navigation via buttonNavigator.  Call from loop() if overriding.
   void handleNavigation();