Add source elements to the pipeline before activation

In plug_src we changed the element state before adding it to
the owner container. This prevented the pipeline from intercepting
a GST_STREAM_STATUS_TYPE_CREATE message from the pad in order
to assign a custom task pool.

Fixes gstreamer/gst-rtsp-server#53
5 jobs for plug-src-before-pipeline-activation in 6 minutes and 24 seconds (queued for 1 second)