Can be used with Callback<T> (means it's still type safe). It's used to
pass not only Request id to user's callback, but also a value user
wanted.
void *data field is removed from RequestInfo.
This commit also adds GlobalFunctionCallback, because it was needed in
order to replace plain C pointers to functions (which were used in
Request) into our object-oriented BaseCallback pointers.
These callbacks can call object's methods, not some global C functions.
DropboxStorage::info2() and DropboxStorage::infoMethodCallback()
demonstrate the idea.
