WBXML parser modified to work with file-backed buffers.
New callback definitions:
> WBXMLCharactersInFileHandler - Exposes WBXMLBuffer* .  Intended to replace WBXMLCharactersHandler when working with file-backed buffers.  @warning: If WBXMLContentHandler_s.WBXMLCharactersHandler is not null entire file buffer will be loaded into buffer memory to satisfy callback.
> WBXMLBufferFilenameProvider - called to specify file name of new file buffer.
> WBXMLBufferFilenameProviderFree - called to free resources of file name obtained from WBXMLBufferFilenameProvider.
New functions:
> wbxml_parser_parse_file() - Same as wbxml_parser_parse() for parsing file inputs instead of memory chunk.