Implement FileDialogLabels

[fluxxcode]

Implemented FileDialogLabels and FileDialog::labels to enable multiple language support.

The following example shows how the default title of the dialog can be displayed in German instead of English.

use egui_file_dialog::{FileDialog, FileDialogLabels};

let labels_german = FileDialogLabels {
    title_select_directory: "📁 Ordner Öffnen".to_string(),
    title_select_file: "📂 Datei Öffnen".to_string(),
    title_save_file: "📥 Datei Speichern".to_string(),
    ..Default::default()
 };

let file_dialog = FileDialog::new().labels(labels_german);