#24 FileUtil's access control is flawed

[Mark Lentczner]

Several of the key FileUtil members are private and only
certain classes are allowed to use them. This was done to
insist that anyone using FileUtil would have to edit
FileUtil.hpp to make such use explicit.

As currently implemented, this has two flaws:
1) FileUtil.hpp has to include things like xmlmedia.hpp,
which pulls in a lot. We could simply forward declare the
friend classes, but that would allow a rouge piece of code to
get basically define thier own implementation of these
classes and get around the private.

2) XMLFileContainer is blessed, so all anyone has to do is
use that!