Validate XML files with XML schema

[Fredrik Ullner]

DC++ does not currently validate that the XML files it reads (DCPlusPlus.xml, Favorites.xml etc) are valid. The XML parser is able to deduce that certain information is invalid, but that's not an optimal solution. The current XML parser cannot validate files so the component needs to be updated to handle it or a new XML parser should be used.

There are XSD files in the source code tree under schemas/ that should be used.