Add and implement interfaces for systems (ISystem, IUpdatableSystem, etc.), move Color to Utils, rename Handlers/Renderers/Backends to System, move input related classes to an Input subfolder.

Voile/Source/Audio/DummyAudioSystem.cs

@@ -0,0 +1,48 @@
+namespace Voile.Audio
+{
+    /// <summary>
+    /// Dummy audio system.
+    /// </summary>
+    public class DummyAudioSystem : AudioSystem
+    {
+        public override void AddBusEffect<T>(T effect, string bus = "Master")
+        {
+            return;
+        }
+
+        public override void CreateBus(string busName)
+        {
+            return;
+        }
+
+        public override float GetBusVolume(string busName)
+        {
+            return 0.0f;
+        }
+
+        protected override void Initialize()
+        {
+            return;
+        }
+
+        public override void SetBusVolume(string busName, float volume)
+        {
+            return;
+        }
+
+        protected override void Shutdown()
+        {
+            return;
+        }
+
+        protected override void Update()
+        {
+            return;
+        }
+
+        public override void PlaySound(Sound sound, float pitch, float volume, string bus = "Master")
+        {
+            return;
+        }
+    }
+}