Store results of Biblatex backend detection and use it in consequent runs

Review Request #106363 - Created Sept. 6, 2012 and submitted - Latest diff uploaded

Eugene Shalygin
As we can detect what backend is used by Biblatex only when it prints it out, we need to store that information for next updates of bibliography

This patch use dirty approach: it just stores this as dynamic property of TextInfo object and uses it afterwards. I believe we must store this information somewhere or parse \usepackage{biblatex}.
Maybe in the future Biblatex will provide some other setup commands for specifiyng backend which will be needed to parse also.
I understand, that dynamic property is not the best place to store it. From the other hand, it is kind of local information in this approach, because property name is not used outside.
Manual testing