[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_event_handler.cpp

@@ -0,0 +1,13 @@
+#include <escher/input_event_handler.h>
+#include <escher/app.h>
+#include <escher/toolbox.h>
+#include <escher/metric.h>
+
+bool InputEventHandler::handleToolboxEvent(App * app) {
+  if (toolbox() != nullptr) {
+    toolbox()->setSender(this);
+    app->displayModalViewController(toolbox(), 0.f, 0.f, Metric::PopUpTopMargin, Metric::PopUpLeftMargin, 0, Metric::PopUpRightMargin);
+    return true;
+  }
+  return false;
+}