Adding a relm widget

First, we'll need these new import statements:

use playlist::Playlist;
use playlist::Msg::{
    AddSong,
    LoadSong,
    NextSong,
    PlaySong,
    PauseSong,
    PreviousSong,
    RemoveSong,
    SaveSong,
    SongStarted,
    StopSong,
};

And then, add the Playlist widget below the toolbar:

view! {
    #[name="window"]
    gtk::Window {
        title: "Rusic",
        gtk::Box {
            orientation: Vertical,
            #[name="toolbar"]
            gtk::Toolbar {
                // …
            },
            #[name="playlist"]
            Playlist {
            },
            gtk::Image {
                from_pixbuf: self.model.cover_pixbuf.as_ref(),
                visible: self.model.cover_visible,
            },
            gtk::Box {
                // …
            },
        },
        delete_event(_, _) => (Quit, Inhibit(false)),
    }
}

There's something different with using relm widgets and gtk widgets. Relm widgets must not contain a module prefix...