Rename wxFileSystemWatcherBase::DoAdd() to AddAny() and make it public.

Make this method public to fix compilation after the recent changes as
traverser classes can't call it otherwise (no idea why did it compile with
g++). And give it a better name just in case people decide to call it even
though it's not documented and described as private in comment (but perhaps we
could make it really public later).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72071 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775