Centralised Media Metadata Handling & General Cleanup

[thomasm789]

This PR includes changes already proposed in #68 and #69

This PR centralises and simplifies media metadata handling, fixing issues like flickering between app icons and media artwork. It improves consistency, respects config flags, and handles key edge cases.

Highlights:

• Replaces scattered logic with a single _update_media_status method.
• Fixes flickering by clearly prioritising metadata: Chromecast -> external artwork -> external metadata -> offline mapping -> fallback.
• External and Chromecast metadata processing are conditional (use_external_metadata, use_chromecast).
• Introduces both YouTube and Movie/TV Show media artwork.
• Handles off-state, homescreen, and standby gracefully.
• General cleanup, clearer structure, less redundancy, better logging.