tracker-extract: ignore subtrack titles for videos

Currently, the resulting nie:title includes titles from the individual
tracks due to both gstreamer not differentiating between
global/container tags and track tags until introducing
gst_discoverer_container_info_get_tags() as new API and tracker-extract
gstreamer backend looping through available tracks and collecting all
tags it could find.

As a result, in cases where there is no title tag in the container, but
there are some title tags in subtitle or audio tracks, they are included
in the nie:title and prevent the file name based fallback title in grilo
and cause the video to show up with nonsense titles in totem.

To fix this, replace gst_discoverer_info_get_tags() with the new API,
and ignore title tags from subtracks if the file is a video.

Fixes https://gitlab.gnome.org/GNOME/tracker-miners/-/issues/202