add description property for plugin feature

Pilz.Plugins.Advanced/PluginFeature.cs

@@ -31,6 +31,10 @@ public abstract class PluginFeature
     /// </summary>
     public virtual string? Name { get; init; }
     /// <summary>
+    /// The (display) description of the feature.
+    /// </summary>
+    public virtual string? Description { get; init; }
+    /// <summary>
     /// The symbol for the feature.
     /// </summary>
     public virtual object? Icon { get; set; }