Use document window as parent for "Save changes?" dialog

This ensures that the message box is centered on its parent window and not the
screen.

Closes #17295.

src/common/docview.cpp

@@ -538,7 +538,8 @@ bool wxDocument::OnSaveModified()
                      GetUserReadableName()
                     ),
                     wxTheApp->GetAppDisplayName(),
-                    wxYES_NO | wxCANCEL | wxICON_QUESTION | wxCENTRE
+                    wxYES_NO | wxCANCEL | wxICON_QUESTION | wxCENTRE,
+                    GetDocumentWindow()
                  ) )
         {
             case wxNO: