feat: add playlist shuffle/unshuffle button

[djr715]

Feature

This pull request adds a button that shuffles and unshuffles a playlist.

Description

• Buttons use shuffle icons provided by the included Fluent/Material icon ttf's.
• Visibility is configurable in modernz.conf:

  # show shuffle Button (default=no)
  shuffle_button=(yes/no)
  

• Positioned on the left side of the "loop" button when visible.

Screenshots

Shuffled

Unshuffled