add replacement for UISplitViewController (fixes #165, fixes #236)

UISplitViewController is too buggy
- issue #165 is a long-standing bug
- issue #236 is a new problem that arose with the addition of Auto Layout
  and iOS 7 support

after several hours of debugging and troubleshooting (not the first
session of this kind), I finally decided to give UISplitViewController
the boot and implement my own split view controller.
fix notifications of child view controllers in several parent view controllers
fix overlay view handling in SplitViewController