Bring the new SplitView into master.

This will not, or at least it isn't supposed to, replace the existing SplitView yet. This is going to take a lot of testing, so I have named the new version SplitView2. Once it is stable, I'll remove the dependency on RBSplitView and replace SplitView with SplitView2.

Please start using SplitView2 and open issues for any bugs. Patches will be hugely, greatly, massively appreciated. This stuff is really difficult to get right.