Debugger: Introduce a tree level above stack frames

This level is meant to take the role of current thread handler
in the long run, allowing per-thread stackviews in each engine.

For now, the additional level holds just a single, invisible
dummy item playing the role of a "current thread".

Change-Id: Ief6131500fc1aa8902f2313038a65840b80b495b
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/watchhandler.h

@@ -101,7 +101,6 @@ public:
     QString watcherName(const QString &exp);
 
     void scheduleResetLocation();
-    void resetLocation();
 
     void setCurrentItem(const QString &iname);
     void updateLocalsWindow();