XML tools plugin: update language.dtd.xml

This update Kate's MetaDTD used by the XML completion plugin.
This file was outdated by 11 years (updated in 2002). That means
that people using this plugin worked with completely outdated
highlighting rules... This is totally awful..

Further infos:
http://new.linuxfocus.org/English/May2002/article201.shtml#201lfindex2

CCMAIL: kwrite-devel@kde.org
CCBUG: 318888