Allow nested IContexts and use it to give extra editor windows a context

The extra editor windows need to have editor manager context, otherwise
shortcuts (like ctrl+tab) do not work in them if e.g. projects mode is
active. Doing this via add/removeAdditionalContexts would be non-trivial
and error prone, so adding a context to the extra window is more
convenient. Since editors themselves already define a context, we need
to allow nesting of contexts.

Change-Id: I244eca53ebd665fd4d8fe7531e8ff701ed0b40b2
Reviewed-by: David Schulz <david.schulz@digia.com>
(cherry picked from commit deff0eb3c7)
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/documentmanager.h

@@ -149,7 +149,7 @@ private slots:
     void checkForReload();
     void changedFile(const QString &file);
     void mainWindowActivated();
-    void syncWithEditor(Core::IContext *context);
+    void syncWithEditor(const QList<Core::IContext *> &context);
 };
 
 /*! The FileChangeBlocker blocks all change notifications to all IDocument * that