feat: previous/next buttons support

[Bellisario]

Currently the implementation of Musicale only handles "Next" and "Previous" keyboard buttons, which are keyboard-specific and could not be available to everyone, so "Next" and "Previous" buttons on the player are needed.

This pull implements the following:

• "Next" and "Previous" buttons on the play bar
• Buttons appear only when needed
• An option in settings lets user disable these buttons if wanted
• Buttons appear/disappear (possibly) with an animation
• Buttons become a bit darker (showing they're disabled) when there's no previous/next song to play
• Right click on "Play Next" button allows switching to previous/next song