Listbox: Support for reset action

We need a way to add a reset button to listboxes' headers.

The reset button should...

• ...have a customizable label to reflect the context the listbox is being used in.
• ...only show up if the listbox has a header. Showing the reset button in an empty header is not allowed.
• ...be hidden if no item is selected in the listbox.

Implementation plan

• Expose an resetButtonLabel prop.
• If the prop is omitted, the button never shows up.
• On click, the cleared event is emitted.
• Left-align the header to match the designs in https://www.figma.com/file/qEddyqCrI7kPSBjGmwkZzQ/branch/5YYrJSrszk6dw9619HHPt9/%F0%9F%93%99-Component-library?node-id=425%3A14.