TreeModel: Rework tree iteration

Use function objects to apply directly during a depth-first
walk of the tree instead of faking a flat container of tree
nodes. Less code, and allows even some non-const operations.

Change-Id: I804ab11df358fe937b40809cbcb772e6f3ff8dc5
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/autotest/testresultmodel.cpp

@@ -26,6 +26,8 @@
 #include "testresultdelegate.h"
 #include "testresultmodel.h"
 
+#include <utils/qtcassert.h>
+
 #include <QFontMetrics>
 #include <QIcon>