LibGUI: Use on_up_pressed/on_down_pressed events in SpinBox

Fixes keyboard increment/decrement of SpinBox values.

After PR SerenityOS#2412 the TextBox class started not propagating arrow key
events to the parent widgets because it handles them itself now.
It also added two new events for these arrow keys, so use them instead
in SpinBox.

Libraries/LibGUI/SpinBox.cpp

@@ -41,6 +41,12 @@ SpinBox::SpinBox()
  else
  m_editor->set_text(String::number(m_value));
  };
+ m_editor->on_up_pressed = [this] {
+ set_value(m_value + 1);
+ };
+ m_editor->on_down_pressed = [this] {
+ set_value(m_value - 1);
+ };
 
  m_increment_button = add<ControlBoxButton>(ControlBoxButton::UpArrow);
  m_increment_button->set_focusable(false);
@@ -88,20 +94,6 @@ void SpinBox::set_range(int min, int max)
  update();
 }
 
-void SpinBox::keydown_event(KeyEvent& event)
-{
- if (event.key() == KeyCode::Key_Up) {
- set_value(m_value + 1);
- return;
- }
- if (event.key() == KeyCode::Key_Down) {
- set_value(m_value - 1);
- return;
- }
-
- event.ignore();
-}
-
 void SpinBox::mousewheel_event(MouseEvent& event)
 {
  set_value(m_value - event.wheel_delta());

Libraries/LibGUI/SpinBox.h

@@ -51,7 +51,6 @@ class SpinBox : public Widget {
 protected:
  SpinBox();
 
- virtual void keydown_event(KeyEvent&) override;
  virtual void mousewheel_event(MouseEvent&) override;
  virtual void resize_event(ResizeEvent&) override;