Fixes #2100.

Override toggleVisible of BookmarkDialog to call update before using toggleVisible of QVLCFrame
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit b506e7db2cbd3cf8c7742efc59a6994fd2719477)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>