Debugger: Simplify removal of outdated L&E items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that are expected to change as outdated, while data arrives, undo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

@@ -131,7 +131,8 @@ WatchData::WatchData() :
     elided(0),
     wantsChildren(false),
     valueEnabled(true),
-    valueEditable(true)
+    valueEditable(true),
+    outdated(false)
 {
 }