Support appium-mac2-driver

[KazuCocoa]

So far, appium-mac2-driver capture a screenshot of the entire screen. But the page source is the AUT. Thus, element highlight does not work well at least.

[eglitise]

I wonder if it's even possible to improve the current mac2 driver integration. Right now I believe it does take a screenshot of the entire screen, but this is because the page source contains not only the application window, but also the macOS menu bar that contains the app-specific options.