Debugger[CDB]: Do not evaluate watch expressions at all.

setError() on them instead (should there be any from
a session mixup). Give the session engine watcher
capabilities and ensure 'Remove watch item' is enabled
for the session engine by checking the debugger state.

Reviewed-by: hjk

src/plugins/debugger/watchhandler.cpp

@@ -704,6 +704,9 @@ QVariant WatchModel::data(const QModelIndex &idx, int role) const
 
         case EngineActionsEnabledRole:
             return engine()->debuggerActionsEnabled();
+
+       case EngineStateRole:
+            return QVariant(int(engine()->state()));
     }
 
     const WatchItem *item = watchItem(idx);