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>
