Notifications, security, and editorial changes

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What does this PR do?

Is this a bugfix or an improvement?;

Both.

Explain the changes this PR brings to the project;

• Fixes notification "Next Song" button.
• Shows album art in notifications.
• Fixes many software vulnerabilities.
• Makes editorial changes to the README.md file.

Mention a issue (adding the link, of course) if there is any relationship;

Closes #40.

Insert some print screen of the result, if it changes the UI.

How to test?

Explain how to test in localhost and/or with the build app (what to do, which songs if necessary, etc).

Standard flow works fine.

Critical points

Is there any part of the code that deserves more attention?;

This repository does not seem maintained. I'd love to see it improve until Spotify implements their own notifications for macOS.

Hopefully I'm not overstepping here, but I'd suggest one of:

• Continue to actively create releases, close/respond to issues, and merge PRs.
• Add me as a contributor, if you like, to this repository.
• Point towards my fork as the maintained version and archive the project.

Any technical debt produced by this code? Something could be done better but it wasn't? Why?.

Not that I can see.

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Codebases are now too different to accept a pull request