Fix positioning of open editors popup

Make it position over the editor root view of the active window (of
the current editor view if the active window doesn't have editor views)
instead of always over the main window.
For this to work we should not make the editor manager parentless in the
editor manager placeholder.

Change-Id: I55d38340939a37960ec619b89f2e768bbfab7f24
Reviewed-by: David Schulz <david.schulz@digia.com>