Update/implement skip flag

[prodigy7]

Implementing skip files as request in #2005

Description

This customisation implements skip files. This makes it possible to move downloaded songs without having to download them again.

Related Issue

#2005

Motivation and Context

If you move downloaded files, they will be downloaded again

How Has This Been Tested?

Screenshots (if appropriate)

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have read the CORE VALUES document
• I have added tests to cover my changes
• All new and existing tests passed