added SetNoExt() as there needs to be a way to remove
the extension. One wonders what SetExt( "" ) is supposed
to do. Maybe adding SetEmptyExt() would be better and
backwards compatible.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@32586 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775