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/stackwindow.cpp

@@ -49,6 +49,10 @@ StackTreeView::StackTreeView(QWidget *parent)
 void StackTreeView::setModel(QAbstractItemModel *model)
 {
     BaseTreeView::setModel(model);
+
+    if (model)
+        setRootIndex(model->index(0, 0, QModelIndex()));
+
     connect(static_cast<StackHandler*>(model), &StackHandler::stackChanged,
             this, [this]() {
         if (!m_contentsAdjusted)