Fixes a number of issues with SC.SplitView:

- Adds layoutDirection property and class-name to SplitDividerView. Hooks up propagation from parent SplitView.
- Fixes bug where SplitView's class-names would update incorrectly when layoutDirection changed. (Previous class was not being removed.)
- Fixes CSS bug where selector hierarchies were insufficiently specific, causing a misapplication of styles in nested SplitViews with different layoutDirections.
- Includes unit tests.