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/debugger/threadshandler.h

@@ -39,6 +39,7 @@ namespace Debugger {
 namespace Internal {
 
 class GdbMi;
+class ThreadItem;
 
 class ThreadsHandler : public Utils::TreeModel
 {
@@ -77,6 +78,8 @@ public:
     void resetLocation();
     void scheduleResetLocation();
 
+    void foreachThread(const std::function<void(ThreadItem *item)> &func);
+
 private:
     void updateThreadBox();