User interface improvements (after migrating gtk4 and libadwaita)

With the migration to Gtk4 & libadwaita we can review the current interface and redesign some parts to improve the user experience and the usability of the application.

GNOME HIG that we should try to follow:

• Use BoxedList for load from DL view, Action rows, Combo rows
• Implement first launch "wizard" as a page in the main stack
• Use PreferencesWindow for preferences
• Use AdwFlap for sidebar
• Apply Adwaita styles to HeaderBar buttons
• Use ColumnView for message table
• Update search bar, use SearchBar widget instead of a custom one
• Move InfoBar to the top
• Keyboard shortcuts, do not use Alt if it's possible
• Use Adw.AboutWindow for about (not released yet)
• Show recent files in placeholder view
• Review app name "GNOME Translation Editor"
• Icon redesign?