Improving the organization of the application

The main function is starting to get bigger, so we'll refactor our code a little to make it easier to update in the upcoming sections and chapters.

First, we'll create a new module called toolbar. As a reminder, here's how to do so:

1. Create a new file: src/toolbar.rs.
2. Add a statement, mod toolbar;, at the top of the file main.rs.

This new module toolbar will start with the import statement and the const declaration:

use gtk::{
    ContainerExt,
    SeparatorToolItem,
    Toolbar,
    ToolButton,
};

const PLAY_STOCK: &str = "gtk-media-play";

We'll then create a new structure holding all the widgets that compose the toolbar:

pub struct MusicToolbar {
    open_button: ToolButton,
    next_button: ToolButton,
    play_button: ToolButton,
    previous_button: ToolButton,
    quit_button: ToolButton...