[unity] ui-toolkit package: Code cleanup, added documentation. See #1943.

spine-unity/Modules/com.esotericsoftware.spine.ui-toolkit/Documentation.meta

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spine-unity/Modules/com.esotericsoftware.spine.ui-toolkit/Documentation/README.md

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+## Spine UI Toolkit Extension [Experimental]
+
+This experimental plugin provides integration of UI Toolkit for the spine-unity runtime. Please be sure to test this package first and create backups of your project before using.
+The plugin adds a `SpineVisualElement` control element to be used with UI Toolkit, providing functionality similar to `SkeletonGraphic` to add your Spine skeletons to a Visual Tree Asset via UI Toolkit's UI Builder.
+
+### Usage
+
+For usage of the `SpineVisualElement` control element, you can check out the included `UI Toolkit Example.unity` example scene and `SpineUIToolkit.uxml` file which can be installed via the `Samples` tab in the Unity Package Manager window.
+The new `SpineVisualElement` can be found in the Unity `UI Builder` window under `Library - Project - Custom Controls` `Spine - Unity - SpineVisualElement`. It can be added to your hierarchy and configured as normal UI Toolkit elements while providing Spine skeleton attributes as well.

spine-unity/Modules/com.esotericsoftware.spine.ui-toolkit/Documentation/README.md.meta

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spine-unity/Modules/com.esotericsoftware.spine.ui-toolkit/Runtime/SpineVisualElement.cs

@@ -39,7 +39,6 @@ namespace Spine.Unity {
 
 	[UxmlElement]
 	public partial class SpineVisualElement : VisualElement {
-		public new class UxmlFactory : UxmlFactory<SpineVisualElement, UxmlTraits> { }
 
 		[SpineAnimation(dataField: "SkeletonDataAsset", avoidGenericMenu: true)]
 		[UxmlAttribute]