Add basic-tutorial-4 from the GStreamer tutorials
https://cgit.freedesktop.org/gstreamer/gst-docs/tree/examples/tutorials/basic-tutorial-4.c
There are however some things that are not so nice (but not related to the example):
- ClockTime is not printed as nice as GST_TIME_FORMAT (add std::fmt::Debug trait on ClockTime?)
- seek_simple does not accept a ClockTime but a i64 for the new position