Gradient editor - Implemented stop create/remove and edit

src/Artemis.UI.Shared/AnimationTimeline/BrushAnimation.cs

@@ -0,0 +1,76 @@
+using System;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Media;
+using System.Windows.Media.Animation;
+
+namespace Artemis.UI.Shared.AnimationTimeline
+{
+    public class BrushAnimation : System.Windows.Media.Animation.AnimationTimeline
+    {
+        public static readonly DependencyProperty FromProperty =
+            DependencyProperty.Register("From", typeof(Brush), typeof(BrushAnimation));
+
+        public static readonly DependencyProperty ToProperty =
+            DependencyProperty.Register("To", typeof(Brush), typeof(BrushAnimation));
+
+        public override Type TargetPropertyType => typeof(Brush);
+
+        //we must define From and To, AnimationTimeline does not have this properties
+        public Brush From
+        {
+            get => (Brush) GetValue(FromProperty);
+            set => SetValue(FromProperty, value);
+        }
+
+        public Brush To
+        {
+            get => (Brush) GetValue(ToProperty);
+            set => SetValue(ToProperty, value);
+        }
+
+        public override object GetCurrentValue(object defaultOriginValue,
+            object defaultDestinationValue,
+            AnimationClock animationClock)
+        {
+            return GetCurrentValue(defaultOriginValue as Brush,
+                defaultDestinationValue as Brush,
+                animationClock);
+        }
+
+        public object GetCurrentValue(Brush defaultOriginValue,
+            Brush defaultDestinationValue,
+            AnimationClock animationClock)
+        {
+            if (!animationClock.CurrentProgress.HasValue)
+                return Brushes.Transparent;
+
+            //use the standard values if From and To are not set 
+            //(it is the value of the given property)
+            defaultOriginValue = From ?? defaultOriginValue;
+            defaultDestinationValue = To ?? defaultDestinationValue;
+
+            if (animationClock.CurrentProgress.Value == 0)
+                return defaultOriginValue;
+            if (animationClock.CurrentProgress.Value == 1)
+                return defaultDestinationValue;
+
+            return new VisualBrush(new Border
+            {
+                Width = 1,
+                Height = 1,
+                Background = defaultOriginValue,
+                Child = new Border
+                {
+                    Background = defaultDestinationValue,
+                    Opacity = animationClock.CurrentProgress.Value
+                }
+            });
+        }
+
+        protected override Freezable CreateInstanceCore()
+        {
+            return new BrushAnimation();
+        }
+    }
+}