Debugger: Remove some uses of semi-global currentEngine()

Make use of recent TreeModel improvements in various
tool views, push more operations into the engine-
owned data models, specifically context menu creation.

Change-Id: I479c97102b9fb81611c6461c6df1cec59295179a
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/threadshandler.h

@@ -38,6 +38,7 @@
 namespace Debugger {
 namespace Internal {
 
+class DebuggerEngine;
 class GdbMi;
 class ThreadItem;
 
@@ -46,7 +47,7 @@ class ThreadsHandler : public Utils::LeveledTreeModel<Utils::TypedTreeItem<Threa
     Q_OBJECT
 
 public:
-    ThreadsHandler();
+    explicit ThreadsHandler(DebuggerEngine *engine);
 
     int currentThreadIndex() const;
     ThreadId currentThread() const;
@@ -82,7 +83,9 @@ private:
     void updateThreadBox();
 
     void sort(int column, Qt::SortOrder order);
+    bool setData(const QModelIndex &idx, const QVariant &data, int role);
 
+    DebuggerEngine *m_engine;
     ThreadId m_currentId;
     bool m_resetLocationScheduled;
     QHash<QString, QString> m_pidForGroupId;