window: Use new async FileDialog API
GTK 4.10 deprecates the old GtkFileChooser* APIs in favor of a
new GtkFileDialog utility. This new utility uses async functions
instead of a response signal, so we can make use of Rust's
async/await patterns.
GTK 4.10 deprecates the old GtkFileChooser* APIs in favor of a
new GtkFileDialog utility. This new utility uses async functions
instead of a response signal, so we can make use of Rust's
async/await patterns.