KMail: Fix scrolling up/down on the message viewer

The new style connection for signals does not support default arguments so

connect(mScrollUpAction, &QAction::triggered,
            q, &Viewer::slotScrollUp);

Connects the version of triggered(bool) with slotScrollUp meaning that slotScrollUp always gets a 0 since the action is never checked.

This breaks the API but the widget is only used internally so i think it's something we can live on.

REVIEW: 125565