Sync Dimensions with DOM changes

[mrflix]

Dimensions works by taking a screenshot of the page via a Chrome API. Thus Dimensions should take a new screenshot whenever something in the viewport changes.

It already does that for scrolling – with a slight delay to take the scroll inertia on OSX into account. However it doesn't take DOM changes into account (e.g. changes of the html via Javascript).

[mrflix]

TODO

• [ ] improve scroll watch by waiting for a scroll-end