Add override of GetElement() to IElement interface, to return AppiumWebElement, as was done in Aquality.Appium.Mobile

[mialeska]

    /// <summary>
    /// Gets current mobile element by specified <see cref="CoreElement.Locator"/>
    /// </summary>
    /// <param name="timeout">Timeout for waiting (would use default timeout from settings by default).</param>
    /// <returns></returns>
    /// <exception cref="OpenQA.Selenium.NoSuchElementException">Thrown if element was not found.</exception>
    new AppiumWebElement GetElement(TimeSpan? timeout = null);