macOS: Fix window state after going out of fullscreen

If you went out of fullscreen with the shortcut or menu item, it would
not restore a previous maximized state, but instead resized the window
to the smaller non-maximized state. So,

1. open Qt Creator and resize to some non-maximized size
2. double-click window title to maximize
3. Window > Enter Full Screen
4. Window > Exit Full Screen

now returns correctly to state 2, not 1.

Change-Id: I076321070d011d73bfe888c0a4a5ade24c1057d6
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/coreplugin/windowsupport.h

@@ -80,6 +80,7 @@ private:
     QAction *m_zoomAction;
     QAction *m_closeAction;
     QAction *m_toggleFullScreenAction;
+    Qt::WindowStates m_previousWindowState;
     bool m_shutdown = false;
 };