refactor: BorderLayerRenderer

[MartinZikmund]

GitHub Issue (If applicable): closes #7621, closes #7110

Replaces #7649

PR Type

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What is the current behavior?

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Copilot Summary

🤖 Generated by Copilot at acf61f2

This pull request improves the Fluent theme for Uno by using a new FauxGradientBorderPresenter control to simulate linear gradient border brushes on various controls across platforms. It also simplifies and harmonizes the theme resources and removes unnecessary platform-specific code. Additionally, it fixes some bugs and enhances some features in the ElevatedView and GtkTextBoxView classes, and adds some tests for potential memory leaks. It also deletes an unused file and cleans up some using directives.

PR Checklist

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• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
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• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
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Other information

Internal Issue (If applicable):

[MartinZikmund]

Closed in favor of #16482