Callout editor: Translations

[rlafuente]

On multilingual callouts, there are a few problems with the translation process:

• there is a new panel below the editor that easily takes up a lot of space if there's more than a couple of strings
• the strings to translate are spread out over slides -- you need to open each slide to edit the translatable parts inside
• the Next/Prev translation repeats over each slide, when it should likely be done at a single point

To address this, we came up with a dedicated section to edit all translatable strings in the callout, since it would often be a dedicated task that shouldn't be intertwined with the callout creation process, and should thus be kept separate. It also gives the user a better notion of the translation progress.