Added getAppIcon method

Thank you @dylancom

Two new methods for get app icon

+ getAppIcon(iconPath: string, done?: (err: Error | {code: string} | null, res: WSData | string | null) => void): void;
+ getAppIconPromise(iconPath: string): Promise<WSData | null>;

Example

    // Get app icon by iconPath which you can get from getAppsFromTV
    control.getAppIcon(
      `/opt/share/webappservice/apps_icon/FirstScreen/${APPS.YouTube}/250x250.png`,
      (err, res) => {
        if (!err) {
          console.log('# Response getAppIcon', res)
        }
      }
    )