gui: yubikey: Use Yubikey's add combo box items

* Use the function in the library.
* This also runs the addition of the items in a separate thread so that
  the UI remains responsive.  May have created issues with thread
  safety.
Yubikey: Use 64 bytes for challenge test

* Some Yubikeys may be configured for 64 byte challenge responses, so
  send 64 bytes to work with these devices.