LibGUI: GDialog should close its nested event loop on window close.

Make GWindow::close() so we can override it in GDialog and quit from the
internal event loop when the window manager tells us to close ourselves.

The dialog will return GDialog::ExecCancel in these situations.

Libraries/LibGUI/GDialog.h

@@ -19,6 +19,8 @@ public:
     int result() const { return m_result; }
     void done(int result);
 
+    virtual void close() override;
+
 protected:
     explicit GDialog(CObject* parent);