Summary:
Deprecate the old GESTrackElement children property handling API.

New APIs:
  * ges_timeline_element_list_children_properties
  * ges_timeline_element_lookup_child
  * ges_timeline_element_get_child_property_by_pspec
  * ges_timeline_element_get_child_property_valist
  * ges_timeline_element_get_child_properties
  * ges_timeline_element_set_child_property_valist
  * ges_timeline_element_set_child_property_by_pspec
  * ges_timeline_element_set_child_properties
  * ges_timeline_element_set_child_property
  * ges_timeline_element_get_child_property
  * ges_timeline_element_add_child_property
  * ges_timeline_element_remove_child_property

Deprecated APIs:
  * ges_track_element_list_children_properties
  * ges_track_element_lookup_child
  * ges_track_element_get_child_property_by_pspec
  * ges_track_element_get_child_property_valist
  * ges_track_element_get_child_properties
  * ges_track_element_set_child_property_valist
  * ges_track_element_set_child_property_by_pspec
  * ges_track_element_set_child_properties
  * ges_track_element_set_child_property
  * ges_track_element_get_child_property
  * ges_track_element_add_child_property

Reviewers: Mathieu_Du

Differential Revision: http://phabricator.freedesktop.org/D40