Add xdg-desktop-portal file chooser

[affederaffe]

What does the pull request do?

This PR implements a file chooser based on the xdg-desktop-portal DBus API.
At the moment, the GTK-based file chooser is still the default as I wanted to get some input on wether it should be removed and replaced or simply optional.

What is the current behavior?

GtkFileChooserDialog is used for file dialogs

What is the updated/expected behavior with this PR?

Optimally, deprecate and remove the GTK-based solution and replace it with this one.

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

None

Obsoletions / Deprecations

Fixed issues

Fixes #8213

[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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[avaloniaui-team]

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