Refactor modals

[LeoRiether]

Now modals receive a lambda that returns an Action. This makes modals infinitely more reusable than the previous ones. To illustrate, this is now possible:

let add_modal = Box::new(
    InputModal::new()
        .style(Style::default().fg(Color::BrightRed))
        .on_commit(|input| Action::AddSong(input))
);

let rename_modal = Box::new(
    InputModal::new()
        .style(Style::default().fg(Color::Blue))
        .on_commit(|input| Action::RenameSongTo(input))
);

No special case handling for each type of modal!

Also, modals now send Action::CloseModal when appropriate, which closes any open modal. Previously each modal had its own match case to close itself...