add reset button to modified fields

this button only appears if a field's value
differs from the original osm value. Resetting
changes the value and adds a history entry.
It does not undo history.

css/app.css

@@ -918,6 +918,18 @@ a.selected:hover .toggle.icon  { background-position: -40px -180px;}
     background: #eef0ff;
 }
 
+.preset-field h4 .modified-icon {
+    opacity: 0.2;
+    display: none;
+    pointer-events: all;
+}
+.preset-field h4 .modified-icon:hover {
+    opacity: 0.5;
+}
+.preset-field.modified h4 .modified-icon {
+    display: inline-block;
+}
+
 .preset-field h4[for*="input-"] {
     border: 1px solid #cfcfcf;
     padding: 5px 10px;

js/id/ui/preset.js

@@ -70,7 +70,17 @@ iD.ui.preset = function(context, entity) {
 
         sections.append('h4')
             .attr('for', function(d) { return 'input-' + d.key; })
-            .text(function(d) { return d.label(); });
+            .text(function(d) { return d.label(); })
+            .append('button')
+                .attr('class', 'fr icon undo modified-icon')
+                .on('click', function(d) {
+                    var original = context.graph().base().entities[entity.id];
+                    var t = {};
+                    (d.keys || [d.key]).forEach(function(key) {
+                        t[key] = original ? original.tags[key] : undefined;
+                    });
+                    event.change(t);
+                });
 
         sections.transition()
             .style('opacity', 1);