String wrapper to convert between QString and lg2

Introducing `Git::Internal::String` wrapper class to simplify conversion from and to QString.

* Should it be exported to the public interface? If so, the GitWrap API should change to take this class instead of a QString.
Ooops, thanks! Hm, should we store a `QByteArray` then instead of a `QString` and do the conversion in the c'tor? Or even store the `const char*`?