OS X: Add action for closing external windows

E.g. editor windows and help windows. There is no automatic system
shortcut for this on OS X.

Change-Id: I27f1208cde0a6f4b1a6952a7988d00a8481a08a4
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/coreplugin/windowsupport.h

@@ -49,6 +49,8 @@ public:
     WindowSupport(QWidget *window, const Context &context);
     ~WindowSupport();
 
+    void setCloseActionEnabled(bool enabled);
+
 protected:
     bool eventFilter(QObject *obj, QEvent *event);
 
@@ -61,6 +63,7 @@ private:
     IContext *m_contextObject;
     QAction *m_minimizeAction;
     QAction *m_zoomAction;
+    QAction *m_closeAction;
     QAction *m_toggleFullScreenAction;
 };