Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

Applications/QuickShow/QSWidget.cpp

@@ -200,10 +200,7 @@ void QSWidget::mousedown_event(GUI::MouseEvent& event)
     m_saved_pan_origin = m_pan_origin;
 }
 
-void QSWidget::mouseup_event(GUI::MouseEvent& event)
-{
-    UNUSED_PARAM(event);
-}
+void QSWidget::mouseup_event([[maybe_unused]] GUI::MouseEvent& event) { }
 
 void QSWidget::mousemove_event(GUI::MouseEvent& event)
 {