LibGUI: Add on_return_pressed function to SpinBox

Userland/Libraries/LibGUI/SpinBox.cpp

@@ -35,6 +35,10 @@ SpinBox::SpinBox()
     m_editor->on_down_pressed = [this] {
         set_value(m_value - 1);
     };
+    m_editor->on_return_pressed = [this] {
+        if (on_return_pressed)
+            on_return_pressed();
+    };
 
     m_increment_button = add<Button>();
     m_increment_button->set_button_style(Gfx::ButtonStyle::ThickCap);

Userland/Libraries/LibGUI/SpinBox.h

@@ -26,6 +26,7 @@ public:
     void set_range(int min, int max, AllowCallback = AllowCallback::Yes);
 
     Function<void(int value)> on_change;
+    Function<void()> on_return_pressed;
 
 protected:
     SpinBox();