Since commit a55dafe341, stream-scoped tags no
longer appeared as top-level tags, introducing a behaviour regression, specially
for MP3 files.
The `gst_discoverer_info_get_tags()` API now returns all tags detected for the
given media, as documented.
A new API is introduced to get container-specific tags,
`gst_discoverer_container_info_get_tags()`. The discoverer tool was adapted to
use it. `gst_discoverer_info_get_tags()` is now deprecated in favor of
`gst_discoverer_container_info_get_tags()` and
`gst_discoverer_stream_info_get_tags()`.
Fixes#759
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1107>
