Enable widgets to change the window's scroll position.

[FrenjaminBanklin]

Currently it's possible for widget engines to change their height on demand, but it is not possible for them to scroll to a specific position - in the case of Enigma, for example, the frame containing the player can be incredibly tall but there isn't a way to scroll automatically up/down due to CORS restrictions. Having an engine method like setHeight that can tell the parent window to scroll to a specific location could potentially solve this problem.