Various improvements

Source/Editor/GUI/AssetPicker.cs

@@ -206,6 +206,11 @@ namespace FlaxEditor.GUI
         /// </summary>
         public event Action SelectedItemChanged;
 
+        /// <summary>
+        /// The custom callback for assets validation. Cane be used to implement a rule for assets to pick.
+        /// </summary>
+        public Func<ContentItem, bool> CheckValid;
+
         /// <summary>
         /// False if changing selected item is disabled.
         /// </summary>
@@ -215,6 +220,8 @@ namespace FlaxEditor.GUI
         {
             if (_fileExtension != null && !item.Path.EndsWith(_fileExtension))
                 return false;
+            if (CheckValid != null && !CheckValid(item))
+                return false;
             if (_type == ScriptType.Null)
                 return true;