Concrete text token class.
Text tokens comprise of regular parsed character data (PCDATA) and raw
character data (from the CDATA sections). Internally, their data is parsed with
all entities expanded. Surprisingly, the text token does have a "tag name"
called #PCDATA, which is how the DTD represents it in permissible child
__construct( $data $data, $line = null, $col = null )
Constructor, accepts data and determines if it is whitespace.