Code completion locking

[Olivier JG]

I've refactored the CPP code completion context a bit as well as
reorganized the locking it does. This should fix the occasional deadlock
during "case |" completion.
However, I also made a change to kdevplatform so that the duchain is no
longer locked when calling "completionItems()", since that makes it
impossible to later acquire the foregroundlock. If any other languages
(PHP, Python) rely on that, they'll need to acquire the lock themselves.
Let me know if this is a problem for any reason, or if I screwed
something up.

Please everyone also get a backtrace if you deadlock during code
completion, as it's probably my fault (even though I thought I thought
it through pretty well). I'll fix it quickly.

-Olivier JG