Brushes - Added brush presets

ColorGradient - Refactored so it implements IList

src/Artemis.Core/Plugins/LayerBrushes/ILayerBrushPreset.cs

@@ -0,0 +1,28 @@
+namespace Artemis.Core.LayerBrushes
+{
+    /// <summary>
+    ///     Represents a brush preset for a brush.
+    /// </summary>
+    public interface ILayerBrushPreset
+    {
+        /// <summary>
+        ///     Gets the name of the preset
+        /// </summary>
+        string Name { get; }
+
+        /// <summary>
+        ///     Gets the description of the preset
+        /// </summary>
+        string Description { get; }
+
+        /// <summary>
+        ///     Gets the icon of the preset
+        /// </summary>
+        string Icon { get; }
+
+        /// <summary>
+        ///     Applies the preset to the layer brush
+        /// </summary>
+        void Apply();
+    }
+}

src/Artemis.Core/Plugins/LayerBrushes/Internal/BaseLayerBrush.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections.Generic;
+using System.Linq;
 using SkiaSharp;
 
 namespace Artemis.Core.LayerBrushes
@@ -70,6 +72,16 @@ namespace Artemis.Core.LayerBrushes
         /// </summary>
         public virtual LayerPropertyGroup? BaseProperties => null;
 
+        /// <summary>
+        ///     Gets a list of presets available to this layer brush
+        /// </summary>
+        public virtual List<ILayerBrushPreset>? Presets => null;
+
+        /// <summary>
+        ///     Gets the default preset used for new instances of this layer brush
+        /// </summary>
+        public virtual ILayerBrushPreset? DefaultPreset => Presets?.FirstOrDefault();
+
         /// <summary>
         ///     Gets or sets whether the brush supports transformations
         ///     <para>Note: RGB.NET brushes can never be transformed and setting this to true will throw an exception</para>

src/Artemis.Core/Plugins/LayerBrushes/Internal/PropertiesLayerBrush.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections.Generic;
+using System.Linq;
 
 namespace Artemis.Core.LayerBrushes
 {