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/pdb/pdbengine.cpp

@@ -240,9 +240,9 @@ void PdbEngine::activateFrame(int frameIndex)
     updateLocals();
 }
 
-void PdbEngine::selectThread(ThreadId threadId)
+void PdbEngine::selectThread(const Thread &thread)
 {
-    Q_UNUSED(threadId)
+    Q_UNUSED(thread)
 }
 
 bool PdbEngine::acceptsBreakpoint(const BreakpointParameters &bp) const