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>
The previous code was storing container children in reverse
addition order, this was mitigated by the fact that track elements
were also stored in reverse order, thus restoring the original
order, but it seems more consistent to preserve order throughout,
the extra cost of append operations is negligible.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1549>
