use gst::glib where applicable

Don't include glib in Cargo.toml if we can use the version re-exported from gst.

I thought that was a good idea, until I noticed that plugins which also use gio or gtk needed an explicit common glib. So, I'm not sure this would work properly in an application that combines a gst-plugin-rs and e.g. gtk3-rs. Unfortunately, my applications are far from being up to date and too large to update quickly.