cpp: Allow merging the results of two imports

* lang/cpp/src/importresult.h, lang/cpp/src/importresult.cpp (class
ImportResult): Add member function mergeWith.

This allows creating a consolidated result of several independent
imports. If the import results to merge considered the same keys, then
not all counts can be consolidated correctly, but the important numbers
like the number of considered keys, the number of imported keys, the
number of unchanged keys and the numbers of new user IDs, subkeys,
signatures and revocations should be correct.

• GnuPG-bug-id: T5951