Migrate to Meta backend

[david-swift]

Steps

• [x] Add your name or username and a link to your GitHub profile into the Contributors.md file.
• [x] Build the project on your machine. If it does not compile, fix the errors.
• [x] Describe the purpose and approach of your pull request below.
• [x] Submit the pull request. Thank you very much for your contribution!

Purpose

Migrating to the Meta project enables using the project in combination with other backends to develop cross-platform apps.

Approach

The PR uses the Meta package. Additionally, further changes are made to modernize the framework:

• Remove child windows. Instead, you can add file dialogs to windows in a similar way you add dialogs.
• You do not have to use Idle yourself. If you update state from a non-main thread, it will automatically wrap the update process with Idle.
• Add support for explicit identifiers for state (part of the Meta package). In some cases, this might be required (example: Idle demo).
• Detect deprecated GTK/Adw functions in order not to use them

Before merging this, I will:

• [ ] Replace storing state values to files by an SQLite implementation
• [ ] Implement strict concurrency and migrate Meta & adwaita-swift to Swift 6