Make the player custom slider accessible via VoiceOver

[defagos]

Description

This PR makes our demo custom player slider accessible via VoiceOver.

Changes made

Self-explanatory. More changes can be expected as part of #578 but the demo will likely be revisited first.

Checklist

• [x] APIs have been properly documented (if relevant).
• [x] The documentation has been updated (if relevant).
• [x] New unit tests have been written (if relevant).
• [x] The demo has been updated (if relevant).