Debugger: Fix display of expandable items in GDB and LLDB

Move common code to dumper.py and debuggerengine.cpp and
fix it there.

Change-Id: I20d91d1aa7400fbdb27938c10cf40c8f6019df0a
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

src/plugins/debugger/watchhandler.h

@@ -128,10 +128,9 @@ private:
 class UpdateParameters
 {
 public:
-    UpdateParameters() { tryPartial = false; }
+    UpdateParameters() {}
 
-    bool tryPartial;
-    QByteArray varList;
+    QByteArray partialVariable;
 };
 
 class WatchModelBase : public Utils::TreeModel