Check if QWidget::setWindowTitle() can be optimized

In a recent trace we noticed that QWidget::setWindowTitle() (called my
MApplicationWindow) needs more than 200ms. In the end it turned out that this was a corner case and normally the
call needs ~15ms.

The problem lies in qstring_to_xtp() in qwidget_x11.cpp. It seems as if the
call to QTextCodec::codecForLocale() does blocking X calls
_XlcResolveLocaleName and _XlcRCreateLocaleDatabase. If X is busy with
something out whole app is blocked.
Do we really need these calls? In the code we find this comment:
// ### If we knew WM could understand unicode, we could use
// ### a much simpler, cheaper encoding...
I guess mcompositor understand unicode, doesn't it?