Add HasFdFlag()
Use it for all tests in all variants of
wxFileDialogs. That way it will be easier
to change again centrally, if required.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42399 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775