Scripting: FindEntityWithName and get script instance

+ Some C# UUID stuff
+ Added some Mono functions
+ ScriptEngine:
  + ID -> ScriptInstance map in ScriptEngine
  + GetManagedInstance

+ Fixed Scene not visible after loading a new scene
  + Renamed OnViewportResize to SetViewportSize for Scene and SceneRenderer

ScriptCore/Math/Vector2.cs

@@ -99,7 +99,12 @@ public struct Vector2
     {
         return !(a == b);
     }
-    
+
+    public float Length()
+    {
+        return (float)System.Math.Sqrt(X * X + Y * Y);
+    }
+
     public override int GetHashCode()
     {
         unchecked