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/qml/qmlengine.h

@@ -93,7 +93,7 @@ private:
     void executeJumpToLine(const ContextData &data) override;
 
     void activateFrame(int index) override;
-    void selectThread(ThreadId threadId) override;
+    void selectThread(const Thread &thread) override;
 
     bool acceptsBreakpoint(const BreakpointParameters &bp) const final;
     void insertBreakpoint(const Breakpoint &bp) final;