Cope with multiple XML input files to handle externals

```
A common use of XML formats is as external files so they can be shared between
projects and still
used with svn or diffed. If you have a project in binary or version-control
format, it is trivial to save
a copy in XML format so it can be parsed by RB2Doxy. However, that fails to
gather all the external
items

The XML document therefore will include many blocks like

rbrwDecoratorList
356968098
MPBHD:Users:andydent:dev:Current ...rbrw:rbrwDecoratorList.xml
00F1D...

```

Original issue reported on code.google.com by `dentaroo@gmail.com` on 12 Oct 2008 at 8:26