Save last use tab folder location

[agourlay]

The path to the folder containing tabs is currently kept in memory to make the loading experience smoother.

We could go further and save this to disk so it survives different application runs.

The location should be adapted depending on the OS running.

See TODO in application.rs

Self {
            song_info: None,
            track_selection: TrackSelection::default(),
            all_tracks: vec![],
            tablature: None,
            tablature_id: container::Id::new("tablature-outer-container"),
            tempo_selection: TempoSelection::default(),
            audio_player: None,
            tab_file_is_loading: false,
            sound_font_file,
            beat_receiver: Arc::new(Mutex::new(beat_receiver)),
            beat_sender: Arc::new(beat_sender),
            file_picker_folder: None, // TODO store last folder used in $user/home/.ruxguitar
        }