Changes to CodeCompletionWorker regarding multithreading

[David Nolden]

Hi, it seems like when CodeCompletionWorker etc. were ported to
kdevplatform, someone completely broke the multi-threading. The result
was that for a year now, we are computing all completion items in the
foreground thread (ouch).

I have fixed this now in kdevplatform and cpp, the other languages
might need a slight adaption. In case you're overriding
CodeCompletionWorker::computeCompletions, then you have to make sure
to match the changed signature of the function you're overriding. The
few other changes should be self-explanatory from the compiler errors.

Now the question is, if we want to backport this to KDevelop 4.2. IMO
the broken multi-threading was a quite serious regression which is
fixed now. The problem is that we're using multi-threading now where
it wasn't used before, which might expose multi-threading bugs we
created within the last year, and which weren't visible without
multi-threading.

Greetings, David