Entity Translation Menu integration

This small glue module adds two fields to the entity translation form that allows to translate the menu title and description associated with the node.

This is basically the functionality thought in #1123610: Menu item translation, and exists here only as a workaround. The module works by forcing all menu entries that belong to nodes (that have translations) to have an "undefined" language. This makes the menu item localizable via i18n_string.

Note that this solution is likely not compatible with the proper menu handling to be implemented in entity_translation, i.e. you likely won't be able to just switch over to the new system when it is in. So use this module only if you know what you are doing.

Seeing as this is a temporary stopgap, I won't release this module at any point.