Manage image retrieval at the player level

[defagos]

Description

This PR moves image retrieval to the player level, making implementation of custom asset loading publishers even simpler.

Changes made

• Replace images with image sources which can be used to provide either an image or a URL (URL requests were considered but will be added later if really meaningful).
• Implement publishers for image sources, chapters and PlayerMetadata so that each of them can published updates of themselves when images are loaded.
• Remove image retrieval from Core Business entirely.
• Update enum style for consistency.
• Add missing image width constant.
• Use default URL sessions (hopefully not needed anymore, see Try to mitigate tests that sometimes fail  #272).

Checklist

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