fix: replace reference to window to permit using in web components

[timblack-NukeDigital]

This change permits click events to be handled by the QuillToolbarDropDown when it is being used in a web component.

Before creating the QuillToolbarDropDown instance, it is also necessary to do the following to get the dropdown label to appear.

    // Monkey patch to work in the shadow DOM
    QuillToolbarDropDown.prototype._addCssRule = (cssRule) => {
      const style = document.createElement("style");
      this.shadowRoot.appendChild(style);
      style.sheet.insertRule(cssRule, 0)
    }