Revert "EditorManager: Fix crash when closing an editor upon activation"

Deleting an editor while currentEditorChanged is apparently not a good
idea.
Use a queued connection instead.

This reverts commits 243a6259618e69b8da9ac2438480c449754f7f77,
447c4ed37f8904ca733d6e6253ad19bb0388f209,
d9602ca550c53e60a5229bfec9539e41fa5a694c and
31034870389a512e3450e32ed990e1622bd6e679.

Change-Id: Iccbee25fb77714963faa6f7184d1f5f53ed348a6
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>