Add a source gstreamer element
Expose error in context
Make it possible to set the source in the error state
Add properties to a stream and use those to get a source-output
Fix signal for new-buffer
Attach the socket source to the thread default mainloop
Make subscribe cancellable.
Propagate state and error in context.
Add bus handler for v4l2 source
Use negotiated properties to set capsfilter in v4l2
Fix subscribe in test-client
