Make sure an XML parser fully supporting UTF-16 is used by JOSM

[Gubaer]

See the discussion on dev and josm-dev:

• there's a problem with XML parsers which don't handle ​UTF-16 correctly. Apparently, they insert duplicates of surrogate code points in OSM keys or values while parsing. After a couple of IO operations even small OSM files/fragments can become very large. In OSM the problem was spotted because of ​gothic code points in name:got-tags.

• Xerces-J 2.6.2 seems to be affected
• Xerces-J 2.9.1 seems to be OK
• ​Woodstox StAX XML parser seems to be OK

JOSM should either ship a compliant parser with it's distribution or check/enforce on startup that a known compliant parser is on the classpath.