Tracing: CtfVisualizer: Add menu to restrict view to certain threads

Add a new dropdown menu to select which threads should be displayed.
If no threads are selected, all are shown (this is why the word
'restriction' was chosen). At the moment this only affects the
timeline view, see the follow up for the statistics view.

Change-Id: Ib3b08ea895e852189156e23feb8dea5f843cceb3
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/plugins/ctfvisualizer/ctftimelinemodel.h

@@ -67,6 +67,8 @@ public:
 
     void finalize(double traceBegin, double traceEnd, const QString &processName, const QString &threadName);
 
+    int tid() const;
+
 signals:
     void detailsRequested(const QString &eventName) const;