Close AUI MDI children when the parent frame is closed

This ensures that the parent frame doesn't close if any of the children
can't be closed and also that all children are closed before the parent
if they do agree to close.

Closes https://github.com/wxWidgets/wxWidgets/pull/1734

Closes #15170.

include/wx/aui/tabmdi.h

@@ -108,6 +108,11 @@ protected:
     virtual void DoGetClientSize(int *width, int *height) const wxOVERRIDE;
 
 private:
+    void OnClose(wxCloseEvent& event);
+
+    // close all children, return false if any of them vetoed it
+    bool CloseAll();
+
     wxDECLARE_EVENT_TABLE();
     wxDECLARE_DYNAMIC_CLASS(wxAuiMDIParentFrame);
 };