Added application setting objects.

Harmonia.WinUI/Session/ApplicationSession.cs

@@ -0,0 +1,10 @@
+namespace Harmonia.WinUI.Session;
+
+public partial class ApplicationSession
+{
+    public WindowSession Window { get; set; } = new();
+    public NavigationSession Navigation { get; set; } = new();
+    public AudioLibrarySession Library { get; set; } = new();
+    public AudioPlayerSession Player { get; set; } = new();
+    public PlaylistSession Playlist { get; set; } = new();
+}