[apps/escher/toolbox] handleEventWithText for all responders.

This makes the code in the various toolbox more generic. The arguments
of the text to insert are each replaced by a Ion::Charset::Empty.
These Empty chars are used to create layouts in ExpressionLayoutFields,
and to compute the position of the cursor in other fields, before being
removed.

Change-Id: Ie81c1e394b06fef2ab801ccff919d6550f70ec30

apps/code/python_toolbox.h

@@ -10,10 +10,7 @@ namespace Code {
 
 class PythonToolbox : public Toolbox {
 public:
-  typedef void (*Action)(void * sender, const char * text);
   PythonToolbox();
-
-  // StackViewController
   bool handleEvent(Ion::Events::Event event) override;
 protected:
   KDCoordinate rowHeight(int j) override;