[escher] Clean Field and Input class hierarchy:

Class hierarchy:
- InputEventHandler
- TextInput
- EditableField derives from InputEventHandler
- LayoutField, TextField and ExpressioField derives from EditableField
- TextArea and TextField derives from TextInput

escher/src/input_view_controller.cpp

@@ -64,7 +64,7 @@ bool InputViewController::textFieldDidReceiveEvent(TextField * textField, Ion::E
   return m_textFieldDelegate->textFieldDidReceiveEvent(textField, event);
 }
 
-Toolbox * InputViewController::toolboxForTextInput(TextInput * input) {
+Toolbox * InputViewController::toolboxForTextInput(InputEventHandler * input) {
   return m_textFieldDelegate->toolboxForTextInput(input);
 }