baseparse: add getcaps function

baseparse: add getcaps function

Adds a getcaps function to the sink pad to make parsers propagate
downstream caps restrictions to upstream.

The pipeline "audiotestsrc num-buffers=100 ! faac ! aacparse !
"audio/mpeg, version=(int)4, stream-format=(string)adts" ! filesink"
wouldn't work because aacparse wouldn't propagate the adts restriction
upstream to faac.

This patch adds a default getcaps to the sink pad to simply proxy
downstream caps and also adds a 'get_sink_caps' function pointer
to GstBaseParseClass for subclasses that need more refined getcaps.

https://bugzilla.gnome.org/show_bug.cgi?id=661874