Closing subdiagram editors closes ALL editors

[tkutz]

Scenario: Open a statechart with a subdiagram state, open that subdiagram and open some additional statechart. Then close the first statechart (the one with the subdiagram state). A dialog asks for confirmation to close all subdiagram editors, however ALL editors are closed.

The expected behavior is that only the subdiagram editors are closed, or even better only the ones related to the actual statechart to be closed.