properly ignore a mouse press if it's going to be context menu event

svn path=/trunk/KDE/kdegraphics/okular/; revision=708268

ui/thumbnaillist.cpp

@@ -581,7 +581,7 @@ void ThumbnailWidget::mousePressEvent( QMouseEvent * e )
 {
     QRect r = m_visibleRect.geometry( m_pixmapWidth, m_pixmapHeight );
 
-    if ( r.contains( e->pos() ) )
+    if ( e->button() != Qt::RightButton && r.contains( e->pos() ) )
     {
         mouseGrabPos = e->pos();
     }