Upgrade `tauri-plugin-dialog` to `gtk4-rs` (potentially updating the upstream crate `rfd`)

[tweidinger]

We need to upgrade from gtk to gtk4-rs to migrate a unsoundness issue found in glib < 0.2.
Please claim this issue if you want to support or work on this and keep in mind this is a prioritized issue and we try to migrate as soon as possible. If the issue is already assigned to someone you can either reach out with comments or questions reach out for collaboration to the assigned person.

This transition includes:

• Upgrading gtk to gtk4-rs and handling all errors caused by the dependency change
• Handle all changes in a backwards compatible way if possible
  • Raise awareness to the working group if breaking changes are required
• Document migration errors that may be issues in the future or which were complicated to handle
• Reach out to the working group (alternatively @tweidinger first) if blockers or upstream issues are hit

A possible alternative would be using the xdg feature instead, at least to unblock us and focus on the other upgrades. We'll lose the message dialogs on Linux but imo that can be a fair tradeoff. Further discussion on this should happen if no one claims and implements the transition including potential work on rfd.