Create method to navigate to Previous/Next items in playlist

[1hitsong]

Changes

Adds previous and next item buttons to OSD, allowing users to navigate through playlists (including behind the scenes playlists, such as TV show seasons)

Also links previous and next buttons in audio player to navigate through items if audio is part of the playlist.

Demo

https://social.linux.pizza/@tgpo/111512062495955328

Multi-part phases

1. Core functionality to navigate through playlists ⬅️ This PR
2. Add UI element showing the title of the next and previous items

[cewert]

Since you labeled this as multi-part, can you give a quick run down of each part you have planned?

[cewert]

Works as expected. Tested with Movies, Videos, Episodes, and Music Videos. Are there more possible types?

IMO when skipping to the next item it would make sense to mark the current item as watched but the behavior in this PR matches the web client 🤷

[1hitsong]

Works as expected. Tested with Movies, Videos, Episodes, and Music Videos. Are there more possible types?

Playlists also support mixing in audio and audiobooks, so they are also supported within this PR.

[cewert]

I forgot I also tested with Audio files. Since audiobooks are basically the same thing I think that covers it 😄

[cewert]

Just noticed something. When hitting previous/next while playing an album, the GUI is paused while the next track is loaded. When hitting previous/next while playing a song in a mixed playlist, most of the GUI goes black while loading the next song. This creates a jarring effect when the next song is from the same album.

[1hitsong]

@cewert See if that's better for you. I added a check so if you're currently playing audio and the item you're navigating to is also audio, it keeps you in the audio player instead of backing out and back in to the player.