EditorManager: Replace another root-to-leaf search by leaf-to-root.

For finding the "parent" SplitterOrView it's much easier to go up the
parent hierarchy than doing a depth-first search from the root.

Change-Id: I4c8e195dae2d1f834fb89ab3c213bad5a0fa7d2a
Reviewed-by: David Schulz <david.schulz@digia.com>