The PrivateStream should keep track of stream tags only. Likewise, the
GstDiscovererInfo should keep track of global tags only.

This patch fixes the issue where the discoverer would report duplicated tag
titles, especially for Matroska media files. The Matroska demuxer emits
correctly-scoped tags, but downstream was making no distinction of them.

Fixes #598, #836, gstreamer/gst-plugins-good#827

Part-of: <gstreamer/gst-plugins-base!1275>