Tags are pushed to "videosrcpad"/"audiosrcpad" in
gst_dvdemux_add_pad() method, however they will be NULL
in this method, hence tags are not pushed.
Instead, send tag event to "pad" created gst_dvdemux_add_pad().
Signal no-more-pads when both pads are created
https://bugzilla.gnome.org/show_bug.cgi?id=743657