Mechanism to modify JOSM settings and store files

[stoecker]

Currently we have a way to access presets and styles, but some of these requires changing configuration. We would need a way (e.g. using a XML format) to modify josm settings:

• adding new entries
• modifying entries
• adding new elements to preferences collections

E.g. Chaning colors, adding new WMS sources or combined presets/mappaint styles would be the target.

Should a bit like the way windows registry edit files work with registry editor.

The system should include a mechanism to store files inside of JOSM's folder structure. Also probably an uninstall mechanism would be a good idea.