Implement tutorial 4 and 5 using safe API

I figured it would be nice to have some kind of roadmap/goals regarding the big piece of API currently missing. Implementing tutorial 4 and 5 sounds like a good target. We already have a start of the latter which will probably be renamed at some point.

So here are the bits we are missing:

• create pod objects and choices: #11
• bindings for spa_video_info()
• wrap Stream::param_changed's param pod attribute to a high level object with API to retrieve the video infos (spa_format_parse() and spa_format_video_raw_parse())
• high level API to handle buffers: #16 (closed)

Anything else missing?