Improve efficiency of tag reading in GStreamer
We currently use the GStreamer 'discoverer' element by default. This decodes media files in order to find their metadata.
It's not the most efficient solution possible, and we attempted to optimize this many years ago: see https://bugzilla.gnome.org/show_bug.cgi?id=564749, and related https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/50 and https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/30.
Fixing this would hopefully allow us to remove our mp3-specific extractor, as well as gaining better media scanning performance for everyone. Although, it's worth profiling to make sure this is really the case.