#31 safe file writing

[Mark Lentczner]

when XMLFileContainer writes a file, it should write to the
side, and only move over when all is okay - and perhaps
there needs to be a way for XMLWriter to signal all okay,
since if it throws in the middle we don't want the file
written over.

[Mark Lentczner]

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This has been implemented and mostly tested as of today.

Only caveat is that the test doesn't check that XMLFileContainers with
sub-containers are properly handled. In fact, in Safe mode they will not
be since in is not possible to rename a dir onto a non-empty dir, and
deleting a dir isn't implemented.

Also, the test in xmlutil-test.cpp, could do a nicer job of cleaning the
directly (in particular, handling non-empty dirs.)