Add optionnal buttons to move on chapters and playlist

[dascritch]

• [x] Previous track in playlist, prevtrack
• [x] Next track in playlist, nexttrack
• [x] Previous chapter (or beginning), prevcue
• [x] Next chapter (or after last chapter if no cue to end), nextcue
• [x] Add an example in user-created panel

[dascritch]

In fact, linked to ticket #108 , and will help to do #122

[dascritch]

To test, we need an "example" page , with a panel with named buttons