code: public BasePanel getSubScreen(BasePanel parentScreen, ScreenLocation screenLocation, Map<String, Object> properties, int screenNo)
      {
          return null;        // Must override
      }
docs: Build this sub-screen.
  @param parentScreen The parent screen.
  @param screenLocation The location to place the new sub-screen (null = same as current sub-screen).
  @param iScreenNo The sub-screen to build.