Add SwitchToFrame and SwitchToRoot methods to PageObject`1

[YevgeniyShunevych]

Support the following iframe switch functionality.

Direct/In-test

Go.To<RegularPage>().
    SwitchToFrame<FramePage>(By.Id("iframe-id")).
        TextBox.Set("abc").
        SwitchToRoot<RegularPage>().
    DoOtherStuff();

Within page objects

public class RegularPage : Page<_>
{
    // Some regular page properties.

    public FramePage SwitchToFrame()
    {
        return SwitchToFrame<FramePage>(By.Id("iframe-id"));
    }
}

public class FramePage : Page<_>
{
    // Some frame page properties.

    public RegularPage SwitchBack()
    {
        return SwitchToRoot<RegularPage>();
    }
}

Go.To<RegularPage>().
    SwitchToFrame().
        TextBox.Set("abc").
        SwitchBack().
    DoOtherStuff();    

Other

Pass an instance of the page object

SwitchToFrame(By.Id("iframe-id"), new FramePage(someArg))

SwitchToRoot(new RegularPage(someArg))

Temporarily navigation

SwitchToFrame<FramePage>(By.Id("iframe-id"), temporarily: true)

SwitchToFrame(By.Id("iframe-id"), new FramePage(someArg), temporarily: true)

Pass an instance of IWebElement:

IWebElement frame = Driver.Get(By.Id("iframe-id"));
SwitchToFrame<FramePage>(frame)