Only allow fields to appear once in the canvas

Listing the same UFField twice within one UFGroup is invalid and produces undefined behavior. The profile editor should prevent this from happening.

Any of these UIs would do the job:

• Remove items from palette if they are already on the canvas.
• Mark items in the palette as gray/strikethrough/inactive if they are already on the canvas.
• When dropping a redundant item, display a warning and skip adding it.
• Mark duplicates in the canvas as invalid (red) and prevent saving until they are resolved.