[Screencast]: Disable context menu

[mliao95]

This PR removes context menu from the screencast since we cannot easily replicate the OS generated context menu from the target.

We disable the context menu in two places:

• from appearing on the screencast canvas by adding a context menu listener that calls event.preventDefault()
• from appearing in the target browser by preventing mouse events with mouseEvent.button === 2 from being emitted to the page.

[mliao95]

Do you all think we should disable the right click button to be emitted to the page as well? I had that change set up initially, but decided against it in case the target page overrides the right click functionality.

The main con is that the target page (not the screencast), will have a context menu open that the user cannot see or interact with from the screencast.

[antross]

Do you all think we should disable the right click button to be emitted to the page as well?

That's my leaning. I don't think I'd expect to find the context menu in the browser, it won't really work for headless, and if I really want it I can interact with the real browser window directly.