Adds RootResponder-level support for data-drag events.

- On dragged files, automatically prevents default (browser leaves app and opens file).
- Adds view responder chain events dataDragEntered, dataDragOver, dataDragDropped and dataDragExited, triggered similarly to mouse events.
- Adds document-level default responder actions dataDragDidEnter, dataDragDidDrop and dataDragDidExit.
Adds default responder event 'dataDragDidHover' for lack of a better name, to allow statecharts to trigger a drop.
Changes view responder chain dataDragOver event to dataDragHover for consistency with default responder events.
Renames dataDragHover event to dataDragHovered event for consistency with other view chain events.
Oh yeah, run loops.