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.
Oh, and given the literal definitions as I wrote them in the previous example, some people also do:

~~~C++
class Foo : private X // or Y for that matter
{
    // cannot be copied, assigned, moved, etc...
};
~~~