Description

This change to the DropdownButton widget allows users to define custom widgets as the icon.

Related Issues

Fixes #25151
Fixes #20686

A huge thank you to the following contributors for inspiration:
@mfiels : #23834
@kushalmahapatro: #26530
@taishi-h: #25198
@MatthewJones517: #28670

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes tests for all changed/updated/fixed behaviors (See Test Coverage).
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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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