Add an option to compute pointerMove coordinates from element top-left

I was talking to somebody about this at TPAC -- I'm pretty sure it was @shs96c, and maybe @AutomatedTester was also there -- but we didn't talk about it during the meeting proper, so I don't think it's in the minutes, and we don't seem to have written it down anywhere else.

The basic proposal was to create a way, for pointerMove actions, to specify that the x and y coordinates should be computed relative to the top-left corner of the element rather than the in-view center point of the element. The main benefit is to provide an easier transition from the Selenium JSON wire protocol, where the existing /moveto command uses the top-left corner.

This was also mentioned in https://github.com/mozilla/geckodriver/issues/789#issuecomment-310139137 but I don't believe a spec issue was raised at that time. The benefit raised in that thread is that coordinates relative to the top-left are much more intuitive for, e.g., canvas elements.

(To be clear: I am not proposing that this be in the Level 1 spec, especially at this point in the process.)

Start with the pointer actions section of the WebDriver specification and review the linked geckodriver discussion for prior context. Determine how an option for top-left-relative pointerMove coordinates would fit the existing specification and JSON Wire Protocol behavior. Done means the proposal has an agreed specification location and clear semantics.