Added audio player logic and tests.

Harmonia.Core/Player/IAudioPlayer.cs

@@ -0,0 +1,28 @@
+using Harmonia.Core.Engine;
+using Harmonia.Core.Playlists;
+using System.ComponentModel;
+
+namespace Harmonia.Core.Player;
+
+public interface IAudioPlayer
+{
+    Playlist? Playlist { get; }
+    PlaylistSong? PlayingSong { get; }
+    double Position { get; set; }
+    RepeatState RepeatState { get; set; }
+    double Volume { get; set; }
+    bool IsRandom { get; set; }
+    bool IsMuted { get; set; }
+    AudioPlaybackState State { get; }
+
+    Task<bool> LoadAsync(int index, PlaybackMode mode);
+    Task<bool> LoadAsync(PlaylistSong song, PlaybackMode mode);
+
+    void Play();
+    void Pause();
+    void Stop();
+    Task PreviousAsync();
+    Task NextAsync();
+
+    event PropertyChangedEventHandler PropertyChanged;
+}