EditorManager: Use static pattern, adjust surrounding code

Change-Id: I3255a0150cd9a730336456c5a9f986eb74fefbff
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/coreplugin/editmode.h

@@ -55,7 +55,6 @@ private slots:
     void grabEditorManager(Core::IMode *mode);
 
 private:
-    EditorManager *m_editorManager;
     QSplitter *m_splitter;
     QVBoxLayout *m_rightSplitWidgetLayout;
 };