Add standalone event property.

Assets/Cryville/Crtr/Chart.cs

@@ -110,7 +110,10 @@ namespace Cryville.Crtr {
 		
 		[JsonIgnore]
 		public abstract int Priority { get; }
-		
+
+		[JsonIgnore]
+		public virtual bool Standalone { get { return false; } }
+
 		public ChartEvent Clone() {
 			return (ChartEvent)MemberwiseClone();
 		}
@@ -417,6 +420,7 @@ namespace Cryville.Crtr {
 #pragma warning restore IDE1006
 
 			public override int Priority { get { return 0; } }
+			public override bool Standalone { get { return true; } }
 
 			public Judge() {
 				SubmitPropSrc("name", new PropSrc.Identifier(() => Id.Key));