Change arrows to point right when closed, down when open

The collapsible button arrows are now more conventional in their
indication of the open/closed state of the box.

Libs/Widgets/ctkCollapsibleButton.cpp

@@ -565,11 +565,11 @@ void ctkCollapsibleButton::paintEvent(QPaintEvent * _event)
     }
   if (d->Collapsed)
     {
-    style()->drawPrimitive(QStyle::PE_IndicatorArrowDown, &indicatorOpt, &p, this);
+    style()->drawPrimitive(QStyle::PE_IndicatorArrowRight, &indicatorOpt, &p, this);
     }
   else
     {
-    style()->drawPrimitive(QStyle::PE_IndicatorArrowUp, &indicatorOpt, &p, this);
+    style()->drawPrimitive(QStyle::PE_IndicatorArrowDown, &indicatorOpt, &p, this);
     }
 
   // Draw Text