Missing property translation (WIP?)

In the current implementation, when using the childTranslationStrategy, an MissingTranslationException is thrown when a new translatable property is added to a Document.

At first I just changed this to use an empty value, but DocumentManagerTest::testFindTranslationWithChildren test began to fail. So I made it optional ($ignoreMissing).
If that's even a good solution, I'm still not sure the default values are okay at this moment (true for childStrategy, false for attributeStrategy)