Add scroll* methods

This is a spot to work on adding the `#scroll*` methods to `Slot`. See #255 and #519. This would allow slots to be scrollable, separately from the app window.

Currently, the methods are implemented at the DSL level, with some specs (but not enough). I got stuck at this point because I think that in order to implement scrolling (and display scrollbars), we need to back each slot with an SWT object. Currently, though, *Slots have no corresponding SWT objects*...they are just logical containers with no "physical" presence.

This would be a major change, so I thought this would be a good place to pause for discussion. I would imagine that it would also affect @jasonrclark's work on text layout. Any thoughts or insights on this? @ashbb @PragTob???