discoverer: Prevent stream tags from leaking in global tags
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 (closed), #836 (closed), gst-plugins-good#827 (closed)