We used to only track the first one but this was wrong, so we start
tracking all the children properties here, adapting the test which
was already thought for this to be implemented.
At the same time add some flags to determine how children properties
need to be handled adding a mode that means that all duplicated
children props will be set together when the user sets that particular
child property. This is going to be tested in a following commit.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/918>
ges-timeline-element property getter handler was using
g_value_take_object() with internal pointers of the element as
arguments, instead of g_value_set_object().
g_value_take_object() moves the ownership of the reference; hence,
when reading "timeline" the reference ownership of timeline is moved
away from the ges-timeline-element and into the GValue.
Since GValues are temporaries that are often discarded quickly after,
this can easily lead to a double free. This was causing
gst-editing-services / pythontests to crash when running
TestTrackElements.test_ungroup_regroup() because of an innocent read of
`clip2.props.timeline` around the end of the test.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4924>
