Added wxTopLevelWindow::MSWGetSystemMenu() method.

Also generate events corresponding to WM_SYSCOMMAND messages for the custom
items of the system menu.

Add a small snippet to test the new functionality to the dialogs sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68596 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/toplevel.h

@@ -77,6 +77,19 @@ public:
     virtual bool CanSetTransparent();
 
 
+    // MSW-specific methods
+    // --------------------
+
+    // Return the menu representing the "system" menu of the window. You can
+    // call wxMenu::AppendWhatever() methods on it but removing items from it
+    // is in general not a good idea.
+    //
+    // The pointer returned by this method belongs to the window and will be
+    // deleted when the window itself is, do not delete it yourself. May return
+    // NULL if getting the system menu failed.
+    wxMenu *MSWGetSystemMenu() const;
+
+
     // implementation from now on
     // --------------------------
 
@@ -214,6 +227,10 @@ private:
     void* m_activateInfo;
 #endif
 
+    // The system menu: initially NULL but can be set (once) by
+    // MSWGetSystemMenu(). Owned by this window.
+    wxMenu *m_menuSystem;
+
     DECLARE_EVENT_TABLE()
     wxDECLARE_NO_COPY_CLASS(wxTopLevelWindowMSW);
 };