trackelement: Add a lookup_child vmethod

trackelement: Add a lookup_child vmethod

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