Add buttons to playback queue flyout for clearing queue and saving current queue to playlist.

As reported in https://github.com/digimezzo/dopamine/issues/323#issuecomment-5572843304. Example:

<img width="1018" height="1480" alt="Image" src="https://github.com/user-attachments/assets/d65ff621-d315-483d-8b06-3d860145f976" />