The "children props" of GES elements are listed incorrectly

[aleb]

For example https://lazka.github.io/pgi-docs/GES-1.0/classes/VideoSource.html#GES.VideoSource suggests that you can use "GES.VideoSource.props.posx" but that is not true, you can only read a property with http://lazka.github.io/pgi-docs/#GES-1.0/classes/TrackElement.html#GES.TrackElement.get_child_property

[lazka]

pushed some improvements