Merge pull request #258 from IIzzaya/drag_and_drop

[Fix] Isolate the D&D EditorWindow Sample

Samples/UIWidgetSample/Editor.meta

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Samples/UIWidgetSample/Editor/DragNDrop.meta

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Samples/UIWidgetSample/Editor/UIWidgetsSample.Editor.asmdef

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+        "Unity.UIWidgets",
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Samples/UIWidgetSample/DragNDrop.meta

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