separate plugin ui & make Pilz.Plugins.Advanced package platform independent

Pilz.Plugins.Advanced/PluginFeature.cs

@@ -3,7 +3,6 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
-using Telerik.WinControls;
 
 namespace Pilz.Plugins.Advanced
 {
@@ -30,7 +29,7 @@ namespace Pilz.Plugins.Advanced
         /// <summary>
         /// The symbol for the feature.
         /// </summary>
-        public virtual RadSvgImage? Icon { get; set; }
+        public virtual object? Icon { get; set; }
         /// <summary>
         /// Sets the prioritization of the feature.
         /// This will be respected on abfragen features and on inserting as items using the extension methods"/>.