Add additional attributes to GPX Export for Nodes

[csytsma]

What steps will reproduce the problem?

1. Create new OSM layer
2. Add at least one Node
3. Add an attribute such as Type, Comment, Source. Any of the attributes as defined in ​http://www.topografix.com/GPX/1/1, which is defined as the schema in the export of the subsequent .gpx file.
4. Export the OSM layer using Export to GPX.

What is the expected result?

Any of the attributes as defined in the XSD definition (​http://www.topografix.com/GPX/1/1) should be exported.

What happens instead?

Only name and description are exported for the nodes in the OSM layer. (See nodesToGpxData() in ​http://josm.openstreetmap.de/browser/josm/trunk/src/org/openstreetmap/josm/gui/layer/OsmDataLayer.java?rev=7414)

Please provide any additional information below. Attach a screenshot if
possible.

Repository Root: http://josm.openstreetmap.de/svn
Build-Date: 2013-09-20 01:34:27
Last Changed Author: Don-vip
Revision: 6238
Repository UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
URL: http://josm.openstreetmap.de/svn/trunk
Last Changed Date: 2013-09-20 00:19:19 +0200 (Fri, 20 Sep 2013)
Last Changed Rev: 6238

Identification: JOSM/1.5 (6238 en) Mac OS X 10.9.4
Memory Usage: 364 MB / 505 MB (193 MB allocated, but free)
Java version: 1.6.0_29, Apple Inc., Java HotSpot(TM) 64-Bit Server VM
VM arguments: [-Xbootclasspath/a:/System/Library/PrivateFrameworks/JavaApplicationLauncher.framework/Resources/LauncherSupport.jar, -Xmx512m, -Dapple.laf.useScreenMenuBar=true]
Dataset consistency test: No problems found

Plugin: ImportImagePlugin (30413)
Plugin: InfoMode (30413)
Plugin: PicLayer (30413)
Plugin: editgpx (30414)
Plugin: geochat (29851)
Plugin: geotools (30349)
Plugin: jts (30277)
Plugin: log4j (30197)
Plugin: mbtiles (v1.2)
Plugin: measurement (29854)
Plugin: opendata (29938)
Plugin: pdfimport (30413)
Plugin: plastic_laf (30197)
Plugin: utilsplugin2 (30015)