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/threaddata.h

@@ -31,28 +31,6 @@
 namespace Debugger {
 namespace Internal {
 
-////////////////////////////////////////////////////////////////////////
-//
-// ThreadId
-//
-////////////////////////////////////////////////////////////////////////
-
-/*! A typesafe identifier. */
-class ThreadId
-{
-public:
-    ThreadId() = default;
-    explicit ThreadId(qint64 id) : m_id(id) {}
-
-    bool isValid() const { return m_id != -1; }
-    qint64 raw() const { return m_id; }
-    bool operator==(const ThreadId other) const { return m_id == other.m_id; }
-    bool operator!=(const ThreadId other) const { return m_id != other.m_id; }
-
-private:
-    qint64 m_id = -1;
-};
-
 ////////////////////////////////////////////////////////////////////////
 //
 // ThreadData
@@ -77,12 +55,10 @@ struct ThreadData
         CoreColumn,
         ComboNameColumn,
         ColumnCount = CoreColumn,
-
-        IdRole = Qt::UserRole
     };
 
     // Permanent data.
-    ThreadId id;
+    QString id;
     QString groupId;
     QString targetId;
     QString core;