Add GstPlayer / GstPlay API

The GstPlayer API is missing in the bindings. In the process of adding it I found that adding documentation of the process could help but I tried several things and I don't know in which order the steps should be done. So far I have tracked the following:

1. In meson.build, add the new API (eg: GstPlayer-1.0) to the --merge-with option in the targetbindinate_gstreamer
2. In generated/meson.build, add the new API namespace (eg: Gst.Player) to the --abi-cs-usings option in the target generate_api
3. Update all: ninja -C build update_all This is where I lost track, I am not sure if the GstPlayer-1.0.gir needs to be copied before or it's done automatically