Encapsulate the code temporarily incrementing wxOpenModalDialogsCount in a
small helper class to make it easier to reuse it elsewhere.
See #14823.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73689 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775