Rename DispatchOnSet file, add constructors without senderID

Svelto.ECS/Dispatcher/DispatchOnChange.cs

@@ -7,6 +7,10 @@ namespace Svelto.ECS
         public DispatchOnChange(int senderID) : base(senderID)
         { }
 
+        public DispatchOnChange() 
+        {
+        }
+
         public new T value
         {
             set

Svelto.ECS/Dispatcher/DispatcherOnSet.cs → Svelto.ECS/Dispatcher/DispatchOnSet.cs

@@ -9,6 +9,12 @@ namespace Svelto.ECS
             _senderID = senderID;
             _subscribers = new WeakEvent<int, T>();
         }
+        
+        public DispatchOnSet()
+        {
+            _senderID    = -1;
+            _subscribers = new WeakEvent<int, T>();
+        }
 
         public T value
         {