Replace GLFW key codes with native Flutter GTK support

[robert-ancell]

Description

Replace the existing key events that are mapped to GLFW key codes with native GTK support in Flutter.

Related Issues

This is the second part of flutter/flutter#57603, once Flutter supports GTK key codes.

Tests

No test changes.

Checklist

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Breaking Change

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[fluttergithubbot]

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Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[stuartmorgan-g]

LGTM