fix: Fixed warnings related to TreeView that would appear in Unity 6000.2

Author: Cammin

Assets/LDtkUnity/Editor/Utility/Tree/LDtkTreeView.cs

@@ -3,6 +3,12 @@
 using UnityEditor.IMGUI.Controls;
 using UnityEngine;
 
+#if UNITY_6000_2_OR_NEWER
+    using TreeView = UnityEditor.IMGUI.Controls.TreeView<int>;
+    using TreeViewItem = UnityEditor.IMGUI.Controls.TreeViewItem<int>;
+    using TreeViewState = UnityEditor.IMGUI.Controls.TreeViewState<int>;
+#endif
+
 namespace LDtkUnity.Editor
 {
     internal abstract class LDtkTreeView : TreeView

Assets/LDtkUnity/Editor/Utility/Tree/LDtkTreeViewLevel.cs

@@ -1,5 +1,11 @@
 using UnityEditor.IMGUI.Controls;
 
+#if UNITY_6000_2_OR_NEWER
+using TreeView = UnityEditor.IMGUI.Controls.TreeView<int>;
+using TreeViewItem = UnityEditor.IMGUI.Controls.TreeViewItem<int>;
+using TreeViewState = UnityEditor.IMGUI.Controls.TreeViewState<int>;
+#endif
+
 namespace LDtkUnity.Editor
 {
     internal sealed class LDtkTreeViewLevel : LDtkTreeView

Assets/LDtkUnity/Editor/Utility/Tree/LDtkTreeViewProject.cs

@@ -1,5 +1,11 @@
 using UnityEditor.IMGUI.Controls;
 
+#if UNITY_6000_2_OR_NEWER
+using TreeView = UnityEditor.IMGUI.Controls.TreeView<int>;
+using TreeViewItem = UnityEditor.IMGUI.Controls.TreeViewItem<int>;
+using TreeViewState = UnityEditor.IMGUI.Controls.TreeViewState<int>;
+#endif
+
 namespace LDtkUnity.Editor
 {
     internal sealed class LDtkTreeViewProject : LDtkTreeView

Assets/LDtkUnity/Editor/Utility/Tree/LDtkTreeViewWrapper.cs

@@ -3,6 +3,12 @@
 using UnityEditor.IMGUI.Controls;
 using UnityEngine;
 
+#if UNITY_6000_2_OR_NEWER
+using TreeView = UnityEditor.IMGUI.Controls.TreeView<int>;
+using TreeViewItem = UnityEditor.IMGUI.Controls.TreeViewItem<int>;
+using TreeViewState = UnityEditor.IMGUI.Controls.TreeViewState<int>;
+#endif
+
 namespace LDtkUnity.Editor
 {
     internal sealed class LDtkTreeViewWrapper : IDisposable