LibGUI: Accept() mouse wheel events in {Spin,Combo}Boxes

Prevents ScrollableContainerWidgets from moving when changing
values in embedded widgets with the mouse wheel.

Userland/Libraries/LibGUI/ComboBox.cpp

@@ -39,6 +39,7 @@ private:
             set_focus(true);
         if (on_mousewheel)
             on_mousewheel(event.wheel_delta_y());
+        event.accept();
     }
 
     virtual void keydown_event(KeyEvent& event) override

Userland/Libraries/LibGUI/SpinBox.cpp

@@ -108,6 +108,7 @@ void SpinBox::mousewheel_event(MouseEvent& event)
     if (event.modifiers() == KeyModifier::Mod_Ctrl)
         wheel_delta *= 6;
     set_value(m_value - wheel_delta);
+    event.accept();
 }
 
 void SpinBox::resize_event(ResizeEvent& event)