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/watchhandler.cpp

@@ -257,6 +257,8 @@ QString WatchData::toString() const
 
     if (isChildrenNeeded())
         str << "children=<needed>,";
+    if (source)
+        str << "source=" << source;
     str.flush();
     if (res.endsWith(QLatin1Char(',')))
         res.truncate(res.size() - 1);