add from-to as an option for display names

Some routes (in particular node networks) have no proper name of their
own, but do have a from and/or to attribute set that gives at least some
description of the route.

Note that these routes often do have the `name` or `note` attribute set
using an explicit "from-to" value, but these attributes are not really
meant for that, so for example the Swiss hiking route tagging now
recommends settings only `from` and `to`, not `name` or `note`.

See e.g. https://wiki.openstreetmap.org/wiki/DE:Switzerland/HikingNetwork#Wanderwegenetz

As an example of such a route, see https://www.openstreetmap.org/relation/13126425

For these routes, setting just `from` or just `to` is also possible (in
particular for node network routes that start at an unnamed
node/guidepost). In this case, the missing attribute is replaced by `?`.

This commit modifies the `relation.nameOrder` preference, adding one
extra special value `from-to` which is handled specially.

Author: matthijskooijman

src/org/openstreetmap/josm/data/osm/DefaultNameFormatter.java

@@ -93,6 +93,7 @@ public static void unregisterFormatHook(NameFormatterHook hook) {
     private static final String[] DEFAULT_NAMING_TAGS_FOR_RELATIONS = {
             "name",
             "ref",
+            "from-to",
             //
             "amenity",
             "landuse",
@@ -451,6 +452,12 @@ private static String getRelationTypeName(IRelation<?> relation) {
     private static String getNameTagValue(IRelation<?> relation, String nameTag) {
         if ("name".equals(nameTag)) {
             return formatLocalName(relation);
+        } else if ("from-to".equals(nameTag)) {
+            String from = trcLazy("from", I18n.escape(relation.get("from")));
+            String to = trcLazy("to", I18n.escape(relation.get("to")));
+            if (from != null || to != null)
+                return (from != null ? from : "?") + "-" + (to != null ? to : "?");
+            return null;
         } else if (":LocationCode".equals(nameTag)) {
             return relation.keys()
                     .filter(m -> m.endsWith(nameTag))