Debugger: Simplify removal of outdated L&E items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that are expected to change as outdated, while data arrives, undo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/watchdata.h

@@ -119,6 +119,7 @@ public:
     bool            wantsChildren;
     bool            valueEnabled;  // Value will be enabled or not
     bool            valueEditable; // Value will be editable
+    bool            outdated;      // \internal item is to be removed.
 
     Q_DECLARE_TR_FUNCTIONS(Debugger::Internal::WatchHandler)
 };