Fix: Core: Implement true escaping for the five predefined XML entities

See:
http://en.wikipedia.org/wiki/List_of_XML_and_HTML_character_entity_references#Predefined_entities_in_XML

Name Character Unicode (decimal) Standard Description
quot " U+0022 (34) XML 1.0 double quotation mark
amp & U+0026 (38) XML 1.0 ampersand
apos ' U+0027 (39) XML 1.0 apostrophe (= apostrophe-quote)
lt < U+003C (60) XML 1.0 less-than sign
gt > U+003E (62) XML 1.0 greater-than sign