gstreamer/tags: Replace TypedValue with direct underlying types

Following https://github.com/gtk-rs/gtk-rs/pull/449

@slomo I do think we need a TypedValue here that owns a SendValue and allows to get a borrow to the underlying T::TagType. Otherwise TagListRef::get<T>() simply isn't implementable in a neat way. This still fails to compile the playbin example because of exactly that: in my hacky solution that preserves the lifetime of SendValue by plainly returning it, no type info is attached (the sole job of TypedValue!) and the application doesn't know that type to get it as when calling .get(). Furthermore T::TagType isn't public so we can't even use that (we don't want it, that's ugly).

Are you sure it was right to remove TypedValue from glib after all?