Debugger: Add infrastructure for message tracepoints.

Debugger: Add infrastructure for message tracepoints.

Add context menu to add a 'message tracepoint', that is,
a trace point that outputs a user message and continues.
Intended to be a replacement for manually inserting
a qDebug() into the code to understand the call sequence.
Yet to be implemented for gdb.

Change-Id: I1c303001421fbbae74d98d15d52718579350db6e
Rubber-stamped-by: hjk
Reviewed-on: http://codereview.qt.nokia.com/756
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>