Adding a new string to a project (useful for vocabularies).

Vocabularies are fake translation projects, in the sense that they are not strings extracted from a program. Their strings are added manually. They serve as a collection/dictionary of interesting, important and frequently used terms and their suggested translations. They can be useful for translators on their everyday job and also can help to discuss and unify the terminology used across different programs.

This branch is about features for adding and deleting strings from a vocabulary project.