Add version check for 2018.x when rendering icons

Author: xsduan

Editor/FolderEditor.cs

@@ -1,4 +1,5 @@
-using UnityEditor;
+#if UNITY_2019_1_OR_NEWER
+using UnityEditor;
 using UnityEngine;
 using UnityHierarchyFolders.Runtime;
 
@@ -72,4 +73,5 @@ private void RenderColorPicker()
             GUILayout.Space(10f);
         }
     }
-}
+}
+#endif

Editor/HierarchyFolderIcon.cs

@@ -1,3 +1,4 @@
+#if UNITY_2019_1_OR_NEWER
 using System;
 using System.Collections;
 using System.Collections.Generic;
@@ -206,3 +207,4 @@ private static void RefreshFolderIcons(int instanceid, Rect selectionrect)
         }
     }
 }
+#endif