项目场景：Qt制作音乐播放器五种播放模式 单曲单次播放 、单曲循环播放、顺序播放、循环播放、随机播放
提示：音乐播放器
五种播放模式 单曲单次播放 、单曲循环播放、顺序播放、循环播放、随机播放
enum QMediaPlaylist::PlaybackMode
The QMediaPlaylist::PlaybackMode describes the order items in playlist are played.
| Constant | Value | Description |
| QMediaPlaylist::CurrentItemOnce | 0 | The current item is played only once. |
| QMediaPlaylist::CurrentItemInLoop | 1 | The current item is played repeatedly in a loop. |
| QMediaPlaylist::Sequential | 2 | Playback starts from the current and moves through each successive item until the last is reached and then stops. The next item is a null item when the last one is currently playing. |
| QMediaPlaylist::Loop | 3 | Playback restarts at the first item after the last has finished playing. |
| QMediaPlaylist::Random | 4 | Play items in random order. |
0.单曲单次播放
1.单曲循环播放
2.顺序播放
3.循环播放
4.随机播放