dropdown_widget: Fix click event being triggered multiple times.

Reset event handlers attached to the dropdown widget and by removing
it from DOM.

User is unlikely to use the same dropdown multiple times, even if
they do, the operation is fast.

web/src/dropdown_widget.ts

@@ -429,6 +429,7 @@ export class DropdownWidget {
                     $(this.widget_selector).trigger("focus");
                 }
                 this.on_hidden_callback(instance);
+                instance.destroy();
             },
             ...this.tippy_props,
         });