Debugger: Simplify removal of outdated L&E items

This removes the need of bookkeeping on the engine side. It's
basically a kind of mark-and-sweep: On update begin mark items
that are expected to change as outdated, while data arrives, undo
that marking, and update end remove all remaining marked items.

Change-Id: I739b84869033d511d5c9a80605c079e87ef4f6a7
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/debugger/watchhandler.h

@@ -197,9 +197,9 @@ public:
     void removeAllData(bool includeInspectData = false);
     void resetValueCache();
     void resetWatchers();
-    void notifyUpdateStarted();
+
+    void notifyUpdateStarted(const QList<QByteArray> &inames = {});
     void notifyUpdateFinished();
-    void purgeOutdatedItems(const QSet<QByteArray> &inames);
 
 private:
     WatchModel *m_model; // Owned.