Don't change MDI children order after showing a file dialog in wxMSW.

Don't use the generic focus saving/restoring code for wxMDIParentFrame in
wxMSW as it already saves and restores the active MDI child on its own and we
should let it do it, as our code could change the active child when restoring
focus if it hadn't been saved correctly previously.

The fact that it is isn't saved is another bug, but even if it is fixed, we
should let MSW MDI implementation handle activation as we can't do it any
better -- but can do worse, as the bug described in #16635 shows.

Closes #16635.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78341 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/mdi.h

@@ -91,6 +91,7 @@ public:
     // Responds to colour changes
     void OnSysColourChanged(wxSysColourChangedEvent& event);
 
+    void OnActivate(wxActivateEvent& event);
     void OnSize(wxSizeEvent& event);
     void OnIconized(wxIconizeEvent& event);
 
@@ -145,6 +146,10 @@ private:
     // return the number of child frames we currently have (maybe 0)
     int GetChildFramesCount() const;
 
+    // if true, indicates whether the event wasn't really processed even though
+    // it was "handled", see OnActivate() and HandleActivate()
+    bool m_activationNotHandled;
+
 
     friend class WXDLLIMPEXP_FWD_CORE wxMDIChildFrame;