Debugger: Streamline ThreadHandler

- Use the TreeItem/data pattern recently introduced with Breakpoints
  to remove the need of keeping track of id/object mapping. Opens
  possibility to have thread groups as intermediate level.
- Use the ThreadHandler directly as model for the thread combobox
  to remove the need of manual combo box updates.
- Move setting current thread from individual engines to central code.

Change-Id: I030e21a4aa5ab30b0efbc84528d9cecf29cbbe30
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/debugger/cdb/cdbengine.h

@@ -80,7 +80,7 @@ public:
     void executeDebuggerCommand(const QString &command) override;
 
     void activateFrame(int index) override;
-    void selectThread(ThreadId threadId) override;
+    void selectThread(const Thread &thread) override;
 
     bool stateAcceptsBreakpointChanges() const override;
     bool acceptsBreakpoint(const BreakpointParameters &params) const override;