Add flip link

EditorHelp.js

@@ -49,6 +49,10 @@ class EditorHelp extends EditorEntryBase {
     this.addLine();
     this.addKey('␣');
     this.addText('Highlight       ');
+
+    this.addLine();
+    this.addKey('f');
+    this.addText('Flip link       ');
   }
 
   addLine() {

EditorLink.js

@@ -63,6 +63,22 @@ class EditorLink extends EditorEntryBase {
     this.elem_.classList.toggle('highlight', highlight);
   }
 
+  flip() {
+    let entries = this.nodes_.getEntries(EditorNode);
+    let fromElem = entries[0].getElement();
+    let toElem = entries[1].getElement();
+    let fromHasFocus = document.activeElement == fromElem;
+    let toHasFocus = document.activeElement == toElem;
+
+    toElem.parentElement.insertBefore(toElem, fromElem);
+
+    if (fromHasFocus) {
+      fromElem.focus();
+    } else if (toHasFocus) {
+      toElem.focus();
+    }
+  }
+
   onKeyDown(e) {
     super.onKeyDown(e);
 
@@ -81,6 +97,12 @@ class EditorLink extends EditorEntryBase {
         e.stopPropagation();
         e.preventDefault();
         break;
+
+      case 'f':
+        this.flip();
+        e.stopPropagation();
+        e.preventDefault();
+        break;
     }
   }