Loading and saving the playlist

We have the ability to create a playlist in our music player, but we cannot save a playlist to a file in order to be able to load one later. Let's add this feature to our project.

We'll save the playlist in the m3u file format and to handle this format, we'll use the m3u crate. So let's add it to our Cargo.toml file:

m3u = "^1.0.0"

Add this line to the main module:

extern crate m3u;

Saving a playlist

We'll start by adding a button to save the playlist. First, we add a field in the MusicToolbar structure for the button:

pub struct MusicToolbar {
    open_button: ToolButton,
    next_button: ToolButton,
    play_button: ToolButton,
    pub play_image: Image,
    previous_button: ToolButton,
    quit_button: ToolButton,
    remove_button: ToolButton,
    save_button: ToolButton,
    stop_button: ToolButton,
    toolbar: Toolbar,
}

And in the constructor, we'll create this button:

impl MusicToolbar {
    pub fn new() -> Self {
        let toolbar = Toolbar::new();
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