Linux.Clipboard: Use a new method to read text

[badcel]

GirCore provides a new async method to read text from a Gdk.Clipboard.

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• New feature or enhancement
• UI change (please include screenshot!)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
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What is the current behavior?

Issue Number: N/A

What is the new behavior?

Other information

Quality check

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• Did you follow the code style guideline as described in CONTRIBUTING.md
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• Did you verify that all unit tests pass
• If necessary and if possible, did you verify your changes on:
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  • macOS
  • Linux

[veler]

It looks good to me, thank you for your help! :D