EditorManager: Get rid of weird logic when finding next view

Removes another algorithm that was starting from the root and took the
whole tree into account.
Instead, make findNextView a method of EditorView, and avoid any
explicit usage of a single root splitter.

Change-Id: I343030521472741a8dfd7134ed16d9beeb10d10a
Reviewed-by: David Schulz <david.schulz@digia.com>