Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download UI Editor Free.
UI Editor v1.3.1
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UI Editor is a decidated editor window that allows you to build user interfaces with ease and speed.
∙ Seperate Editing Window
∙ Completely removes UI from 3D scene
∙ Context Menu
∙ Alignment Tools
∙ Arrow Keys Nudging
∙ Grid and Snapping
∙ Drag & Drop controls toolbox
∙ Drag & Drop images from project folder
∙ Edge Snapping
∙ World Background Rendering
∙ Improved Workflow
∙ Full Source Code Included
For questions and support please visit the unity forum thread.