Add undo/redo keys

Editor.js

@@ -194,6 +194,18 @@ class Editor extends List {
         }
         return;
 
+      case 'u':
+        history.back();
+        e.stopPropagation();
+        e.preventDefault();
+        return;
+
+      case 'U':
+        history.forward();
+        e.stopPropagation();
+        e.preventDefault();
+        return;
+
       case '?':
         if (this.addHelpAfter()) {
           e.stopPropagation();

EditorHelp.js

@@ -53,6 +53,16 @@ class EditorHelp extends EditorEntryBase {
     this.addLine();
     this.addKey('f');
     this.addText('Flip link       ');
+
+    this.addLine();
+    this.addKey('u');
+    this.addText('Undo            ');
+
+    this.addLine();
+    this.addKey('⇧');
+    this.addText('+');
+    this.addKey('u');
+    this.addText('Redo      ');
   }
 
   addLine() {

EditorLabel.js

@@ -13,7 +13,7 @@ class EditorLabel extends EditorEntryBase {
     this.listen(this.input_, 'blur', (e) => this.onBlur(e));
     this.elem_.appendChild(this.input_);
 
-    this.lastSnapshotLabel_ = null;
+    this.lastSnapshotLabel_ = '';
   }
 
   afterDomAdd() {

EditorNode.js

@@ -12,7 +12,7 @@ class EditorNode extends EditorEntryBase {
     this.listen(this.input_, 'blur', (e) => this.onBlur(e));
     this.elem_.appendChild(this.input_);
 
-    this.lastSnapshotLabel_ = null;
+    this.lastSnapshotLabel_ = '';
 
     if (label) {
       this.setLabel(label);