[Escher] Responder can get a pointer to the parent App

We assume the root parent is the App. This works well because each view
controller is a Responder.

Change-Id: I5d90053949a4ffbdb78746cc85430a15138e48e7

apps/graph/list/function_cell.cpp

@@ -2,6 +2,7 @@
 
 FunctionCell::FunctionCell() :
   ChildlessView(),
+  Responder(nullptr),
   m_focused(false),
   m_even(false)
 {