fun addPlugin(name: String, atts: Map<String, String>, downloadUrl: URL): Unit
Add a plugin that should appear in the list
name - the name of the plugin *.jar file (including file extension), e.g. MyAwesomePlugin.jar
atts - the main attributes of the plugin manifest, e.g. supplied by JosmManifest.createJosmPluginJarManifest
downloadUrl - the URL from which the plugin can be downloaded