[discoverer] Avoid decoding where possible
Submitted by Mark Nauwelaerts
Assuming tagreadbin as (give or take) in bug 564749 ever makes it into -base, some minor adjustments to discoverer allow using tagreadbin (in turn using a.o. uridecodebin) rather than uridecodebin directly.
While in this mode not all features may be extracted (e.g. no stream topology), it brings some tagreadbin advantages, e.g. typically no decoders are involved.
Patches available in http://cgit.collabora.com/git/user/manauw/gst-plugins-base.git/log/?h=tagreading