MessagesManager implementation

info.magnolia.cms.i18n.MessagesManager is the component that has been used to handled i18n up until Magnolia 4.5. It has its flaws, and could be rewritten.

• it tries to cover too many use-cases (either enforce passing a Locale, or never pass one)
• its responsibilities are not clear - it observes and holds some i18n config (but not all?) and at the same time provides translation support
• DefaultMessagesManager is still very tied to using property files.
• DefaultMessagesManager isn't cleanly decoupled from the system - it's still using content2bean "manually", etc; it's not a real "component".
• rething package name and/or class name (package mixes i18n for UI and for content, to start with...)
• It's not easy to test
• Some logic is burried in the MessagesChain class - which is where, for instance, we've been appending the ??? when a key wasn't found so far. Except that we've seen workarounds popup here and there to remove those, etc..
• info.magnolia.cms.i18n.MessagesUtil proposes too many methods; as a result, we're completely inconsistently use those in many places in our code. Get rid of this.
• http://jira.magnolia-cms.com/issues/?jql=text%20~%20%22messagesmanager%22%20and%20resolution%3DUnresolved

Existing uses and inconsistencies

See #status.

Proposal

This concept and proposal focuses on in-configuration translations. Hopefully, the concepts can be applied to in-code translations. In-template translations will be taken into account (i.e facilitate the maintenance of the corresponding message files), but changing the mechanism they use might be considered a "next step".