XDoc: recognize more invalid characters
StringUtil: do not encode characters that would become invalid values in XML/XHTML
IDreamActivityDescription: fixed compiler warning
XDoc: optimized RemoveInvalidXmlChars() for the case where there are no illegal characters
Merge pull request #117 from bjorg/DR-129

DR-129 XDoc ToString/ToCompactString/ToPrettyString/WriteTo/ToXHtml can emit illegal XML chars