Support desc filter

[Nyholm]

We save the desc filter when exporting, and we do also fallback to that filter when the current translation is empty...

Hm... our desc filter works likes default. Not sure if this is correct.

This will fix #153

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I chose to not reuse the TranslationFilter in the php-translation/extractor because I would like the bundle to be decoupled from the extractor as much as possible.

[mvrhov]

It should not work as a default. In debug mode it should show the content of desc, if trans returns what was passed to it (This means, that there is no translation for that string). In production everything should behave like there is no desc filter, to not slow down the production e.g the call to desc filter should be removed.

[Nyholm]

But then we need to use a custom Translator, right?

[mvrhov]

JMS uses a custom Node visitors, to add / remove code when compilation is done.