playbin: Proxy sink messages if we activate a sink in playbin already

This makes sure the application gets any context related messages and
can do whatever is required to a) get the sink a context or b) share
the context with other elements in the pipeline.

The proxying is necessary because the sink is not a child element of
playbin, but instead will at a later point be a child of some bin
inside playsink.

https://bugzilla.gnome.org/show_bug.cgi?id=700967
playbin: Proxy the playbin context to the sinks
playbin: Improve autoplugging of decoder/sink combinations by trying to activate the sink

And if that fails don't bother autoplugging that sink. Also gives
us more accurate sink caps.
playbin: When activating a fixed sink, proxy error messages too

If activating a fixed sink fails, everything will fail later anyway
and we can just error out early.
adder: Reject segments that have a different rate than the output segment

adder does no rate conversion.
rtspconnection: Make sure to set a sensible default port for the GSocketConnection

Otherwise it will connect to port 0 if no port is given in the URI.

https://bugzilla.gnome.org/show_bug.cgi?id=701798
tests: add more unit test for playbin

Add unit test for autoplugging of video_decoder/video_sink combination
based on capsfeatures.
videoconvert: Fix leaking of the chroma resample helper objects
playbin: Emit {audio,text,video}-changed signals when pads are removed

https://bugzilla.gnome.org/show_bug.cgi?id=702195