Special case multipolygon preset icon

Multipolygons have geometry === "area", so normally
wouldn't trigger a 'preset-icon-relation' class.

This preset usually matches only in the case where
the tags are on the outer way and the multipolygon
itself is untagged. Otherwise, a more specific area
preset will match.

data/presets/presets.json

@@ -4489,6 +4489,7 @@
     },
     "type/multipolygon": {
         "geometry": [
+            "area",
             "relation"
         ],
         "tags": {

data/presets/presets/type/multipolygon.json

@@ -1,5 +1,6 @@
 {
     "geometry": [
+        "area",
         "relation"
     ],
     "tags": {

js/id/ui/preset_icon.js

@@ -16,9 +16,12 @@ iD.ui.PresetIcon = function(geometry) {
                 var icon = preset.icon || fallbackIcon,
                     klass = 'feature-' + icon + ' preset-icon';
 
-                icon = iD.data.featureIcons[icon];
-                if (icon && icon[geometry]) {
+                var featureicon = iD.data.featureIcons[icon];
+                if (featureicon && featureicon[geometry]) {
                     klass += ' preset-icon-' + geometry;
+                } else if (icon === 'multipolygon') {
+                    // Special case (geometry === 'area')
+                    klass += ' preset-icon-relation';
                 }
 
                 return klass;