Debugger: Work on WatchModel performance

Don't instantiate repeating boilerplate item data in some
cases (such as large arrays).

This makes it necessary to access parent WatchItems in
a lot more cases than before and needs another separation of
WatchItem/WatchModel code to keep the dumper autotests
in a functional state.

For a plain std::vector<int> with 1 mio items this reduces
 extraction time from more than 2 minutes to about 3 seconds.

Change-Id: I175c5f6ee90434a6e85342d8bb71bd10a04dd271
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/debugger/debuggerengine.h

@@ -63,7 +63,6 @@ class DebuggerEnginePrivate;
 class DebuggerPluginPrivate;
 class DisassemblerAgent;
 class MemoryAgent;
-class WatchData;
 class WatchItem;
 class BreakHandler;
 class LocationMark;