CDB: Fix stepping and a crash on dumping expanded pointer items.

Simplify & Generalize stepping code, introduce stubs for reverse
debugging. When user switches to another thread than the
event-triggering one in a stopped inferior, explicitly tell
the engine to continue that one.

Improve fixDumperResult to fix missing types (out of scope items)
and call it from expandPointerDumpable.

src/plugins/debugger/cdb/cdbdebugengine.h

@@ -124,6 +124,7 @@ private:
     bool evaluateExpression(const QString &expression, QString *value, QString *type, QString *errorMessage);
     void evaluateWatcher(WatchData *wd);
     QString editorToolTip(const QString &exp, const QString &function);
+    bool step(unsigned long executionStatus);
 
     CdbDebugEnginePrivate *m_d;