Feature: hscroll_indicator (#753)

This is the symetrical of `vscroll_indicator`.

Requested by @ibrahimnasson.

Fixed:https://github.com/ArthurSonzogni/FTXUI/issues/752

src/ftxui/dom/frame.cpp

@@ -37,7 +37,6 @@ class Select : public Node {
   }
 };
 
-
 class Focus : public Select {
  public:
   using Select::Select;
@@ -143,7 +142,6 @@ class FocusCursor : public Focus {
   Screen::Cursor::Shape shape_;
 };
 
-
 }  // namespace
 
 /// @brief Set the `child` to be the one selected among its siblings.