Support Forward and Backward mouse buttons in presentation mode

REVIEW: 130172

ui/presentationwidget.cpp

@@ -702,8 +702,14 @@ void PresentationWidget::mousePressEvent( QMouseEvent * e )
 
         m_goToNextPageOnRelease = true;
     }
-    // pressing right button
-    else if ( e->button() == Qt::RightButton )
+    // pressing the "move forward" mouse button: unlike the left button this
+    // always means "show next page", so we unconditionally delegate to that
+    // action on mouse button press
+    else if ( e->button() == Qt::ForwardButton ) {
+        slotNextPage();
+    }
+    // pressing right or backward button
+    else if ( e->button() == Qt::RightButton || e->button() == Qt::BackButton )
         slotPrevPage();
 }