Translation status values for 2.0

[Yves Savourel <ysavourel@translate.com>]

Talking about attributes: one information that is important and may be useful to have as part of the core is the status of the translation.
In 1.2 for example we have a approved in <trans-unit> and state in <target>.
I think this need to be re-work in a way that allows to know the status of the segment via one attribute.

Looking at the different tools and their usage of state, it seems it varies a lot. But this is one specific list of values that is going to be important to get carefully defined so it has some logical progression but covers as much as possible the states required by the different tools. Each value needs to be carefully defined so misue risks are reduced. We may also want to talk directly to developers and see how they map their values to it.

-ys