implement some window APIs

Implement some window APIs: `browser.switchToDefaultFrame()`, `browser.switchToFrame(id)`, `browser.switchToDefaultWindow()`, `browser.switchToWindow(name)`.

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Depends on https://github.com/groupon-testium/webdriver-http-sync/pull/14
Does this also work for `<a href="/foo.html" target="my-window">Click me</a>`?