Allow to restore the selection by setting the value.

tp-multi-toggle.js

@@ -62,6 +62,16 @@ class TpMultiToggle extends FormElement(LitElement) {
     }
   }
 
+  set value(val) {
+    this.querySelectorAll('tp-button').forEach(el => {
+      if (el.getAttribute(this.attr || 'id') === val) {
+        el.setAttribute('selected', '');
+      } else {
+        el.removeAttribute('selected');
+      }
+    });
+  }
+
   constructor() {
     super();
     this.boundClick = this.onClick.bind(this);