This method can be used for subclass to override the default behaviour
for child lookup. This vmethod can be used for example in the case where
you want the name of a child property to be 'overridden'.
As an example in the GESTitleSource where we have a videotestsrc
which has a 'foreground-color' property that is used in the TitleSource
to set the background color of the title, this vmethod is now used to
tweak the name passed as parameter to rename "background" to
"foreground-backend" making our API understandable.
API:
GESTrackElement::lookup_child
https://bugzilla.gnome.org/show_bug.cgi?id=727880
Most of the time the user should not create GESTrackElements
himself, instead he should add a GESAsset to a layer, that will
result in a clip creation and the proper TrackElements to be
created and added to the tracks.
The case of effects and overlays is a bit different as the user should
create the TrackElement and add them to a clip.
