Custom meta field translation UIs

For particular use cases, like Alley's Fieldmanager repeated fields (see [slideshow code example](http://fieldmanager.org/)), the standard Babble meta field translation UI control will not work as the data is serialised into a single post meta field. In this case users can extend the abstract `Babble_Meta_Field` class and create their own translation UI control for their own particular use cases.

There are some things we need to join together and fix up to allow this
- [x] Allow HTML on the output (i.e. "to be translated") side of the translation UI control
- [x] Allow the `Babble_Meta_Field::update` method to alter the value before it is saved
- [ ] Sync untranslated elements of the serialised data (devs could do it themselves, or ignore it, or we might look to see if we can make this easier…)
- [ ] Tests where appropriate