Slots for styling

At the top level, we could have `slots` that map components if you want to provide custom ones.

```tsx
function MyCustomThumbnailWrapper({ children }) {
  //...
  return <div className="my-classname">{children}</div>
}

<ThumbnailPanel 
  slots={{
    thumbnailWrapper: MyCustomThumbnailWrapper,
    // ..
  }}
/>
```