Split up WP_Digest_Message in smaller chunks

[swissspidy]

The WP_Digest_Message class should be split up in smaller chunks.

Instead of calling class methods for each section and section message, various other classes should hook into new filters inside process_event_items to provider their own message.

This makes it easier for third-parties to "register" custom notifications.

[swissspidy]

To do:

• Tests
• Perhaps use namespaces
• Only add one "Updates" section heading for core update entries. See also Send notifications for available plugin and translation updates #7