Fixed position of context menu in ArtistView and TrackView

- using viewport()->mapToGlobal() as QTreeView is subclass of
  QAbstractScrollArea

src/libtomahawk/playlist/artistview.cpp

@@ -384,7 +384,7 @@ ArtistView::onCustomContextMenu( const QPoint& pos )
     m_contextMenu->setArtists( artists );
     m_contextMenu->setAlbums( albums );
 
-    m_contextMenu->exec( mapToGlobal( pos ) );
+    m_contextMenu->exec( viewport()->mapToGlobal( pos ) );
 }
 
 

src/libtomahawk/playlist/trackview.cpp

@@ -567,7 +567,7 @@ TrackView::onCustomContextMenu( const QPoint& pos )
     }
 
     m_contextMenu->setQueries( queries );
-    m_contextMenu->exec( mapToGlobal( pos ) );
+    m_contextMenu->exec( viewport()->mapToGlobal( pos ) );
 }