Calculator: Allow system shortcuts

Before the key events for shortcuts such as ALT-F4 did not work as the
widget was swallowing up the shortcut. This changes ignores the event
if nothing else matched

Userland/Applications/Calculator/CalculatorWidget.cpp

@@ -184,6 +184,8 @@ void CalculatorWidget::keydown_event(GUI::KeyEvent& event)
         m_divide_button->click();
     else if (event.code_point() == '%')
         m_percent_button->click();
+    else
+        event.ignore();
 
     update_display();
 }