See commit description for changes

Added PageContainer to dynamically load pages within the main content. Added player component to control music playback(Half functional). Added playback changed event not the player service.

Aurora/Backend/Services/PlayerService/PlaybackStateChangedEvent.cs

@@ -0,0 +1,18 @@
+using System;
+
+namespace Aurora.Backend.Services.PlayerService
+{
+    public delegate void PlaybackStateChangedEventHandler(object source, PlaybackStateChangedEventArgs e);
+
+    public class PlaybackStateChangedEventArgs : EventArgs
+    {
+        public PlaybackState OldState { get; }
+        public PlaybackState NewState { get; }
+
+        public PlaybackStateChangedEventArgs(PlaybackState oldState, PlaybackState newState)
+        {
+            OldState = oldState;
+            NewState = newState;
+        }
+    }
+}