Add new functionality to ClipboardExample to demo more of Clipboard API

- Enable paste buttons based on isSupportedType of each type of
  transfer (for API `Clipboard.getAvailableTypes`)
- Copying multiple types of transfers at once (for API
  `Clipboard.setContents` with more than one array entry)
- Clearing clipboard contents (for API `Clipboard.clearContents`)

Part of #2126

Author: jonahgraham