Setting a thread's object name after calling start() is a data race.

QThreadPrivate::start() uses the thread's objectName() to set the thread
name in pthread (PR_SET_NAME), so we must provide it before calling start.
Given that AkThread calls start in the constructor, this means passing
the string as a constructor argument.

While at it, simplify code by using that string for both the object name
of the AkThread (QObject) and the object name of its QThread.

Detected by clang's thread-sanitizer.

CCMAIL: dvratil@kde.org