transport: Move valve from before the decoder to afterwards

If we have the valve before the decoder we will drop all
encoded frames until the stream is linked, including the
initial keyframe usually. So we have to request a new
keyframe which might or might not work but in any case
will take some extra time and delay the initial setup
of the pipeline.
owr: Constify some static arrays
transport: Use the highest ranked, working decoder/encoder for VP8/H264 from the registry

This allows us to use existing codecs on the user's system, especially it
allows to use hardware codecs and removes a dependency on openh264.
build: Requires gstreamer-app-1.0 and gstreamer-video-1.0 too
Allow building with non-static GStreamer plugins
More workaround for a non-static build
build: One more nonstatic build fix

Enables loading of libopenwebrtc.so when the Owr typelib is dynamically
loaded.
More fixes for non-static linking