Use repository-based resource bundle provider instead of plugin translators

Plugin translators, where they are used will be deprecated. Warning messages will be issued when a plugin translator is used.
There seem to be four areas where plugin translators are used that we must treat as API:

• cms validators
• browse perspective sections (documents, assets, images, configuration, etc)
• translation locales (locales that are available for translating content in)
• the workflow menu can fall back on a translation obtained where the workflow is defined (/hippo:configuration/hippo:workflows/category)

There is probably developer documentation that needs to be updated for at least the first three areas here. It is also probably doable to provide an updater script to move these translations.