Add clear hex contents dialog

public/index.html

@@ -358,6 +358,16 @@
     </form>
   </dialog>
 
+  <dialog id="clear-hex">
+    <p>Clear hex contents?</p>
+    <div>
+      <form>
+        <button value="cancel" formmethod="dialog">Cancel</button>
+        <button value="confirm" formmethod="dialog">Confirm</button>
+      </form>
+    </div>
+  </dialog>
+
   <input type="file" accept="image/svg+xml" />
 
   <script src="index.js"></script>