Add keyframes editor to proxy keyframes from subtracks on object track

#519

Source/Engine/Animations/Curve.cs

@@ -378,6 +378,12 @@ namespace FlaxEngine
             {
                 return Time > other.Time ? 1 : 0;
             }
+
+            /// <inheritdoc />
+            public override string ToString()
+            {
+                return Value?.ToString() ?? string.Empty;
+            }
         }
 
         /// <summary>
@@ -655,6 +661,12 @@ namespace FlaxEngine
             {
                 return Time > other.Time ? 1 : 0;
             }
+
+            /// <inheritdoc />
+            public override string ToString()
+            {
+                return Value?.ToString() ?? string.Empty;
+            }
         }
 
         /// <summary>