Implement key Delete in component > Input.

The BS  key is used to delete the character on the left of the cursor.
The DEL key is used to delete the character on the right of the cursor.

BackSpace was already implemented. This CL implements DEL.

This is related to bug:
https://github.com/ArthurSonzogni/FTXUI/issues/4

src/ftxui/component/input.cpp

@@ -52,6 +52,14 @@ bool Input::OnEvent(Event event) {
     return true;
   }
 
+  // Delete
+  if (event == Event::Delete) {
+    if (cursor_position == int(content.size()))
+      return false;
+    content.erase(cursor_position, 1);
+    return true;
+  }
+
   // Enter.
   if (event == Event::Return) {
     on_enter();