Fix method call for updates when items are changed.

tp-dropdown.js

@@ -348,6 +348,10 @@ class TpDropdown extends BaseElement {
       this._focusChanged(this.focused, changes.get('focused'));
     }
 
+    if (changes.has('items')) {
+      this._itemsChanged();
+    }
+
     if (changes.has('value')) {
       this._valueChanged();
       this.dispatchEvent(new CustomEvent('value-changed', { detail: this.value, bubbles: true, composed: true }));