Cpp code completion bugfix and refactor

[David Nolden]

Looks very good, especially all the new tests. Thanks. :-)

I also like the refactoring and added consistency.

I hope all tests pass? If you created new tests that don't pass yet,
they should be QEXPECT_FAIL (I've seen some comments in the test code
saying that something is not working right yet). Given the huge amount
of changes, my only worry is possible (temporary) instability. Maybe
it would be good if you gave it a test ride for a few days, but in
general, feel free to commit it to master.

Greetings, David