Remove the glib_object_get_type! macro

[sdroege]

This macro can be merged with the glib_object_subclass! macro so that
there's less overall boilerplate required.

Also fix-up some documentation.

[sdroege]

Once this is merged, I'll fix-up the examples repo and the gstreamer bindings

[sdroege]

Diff in the examples is btw 2 files changed, 19 deletions(-)

[sdroege]

@GuillaumeGomez all green, can you merge? :)

[EPashkin]

@sdroege Thanks, 👍

[GuillaumeGomez]

Sure, thanks!