Add support for symlinks to wxFileName.

Add support for symlinks to wxFileName.

Allow to work with the symlinks themselves and not the file they reference by
calling the new wxFileName::DontFollowLink().

Update Unix wxDir implementation to not treat symlinks to directories as
directories, this ensures that we don't recurse into the directories outside
of the original parent accidentally.

Closes #14542.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72680 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775