command-line-formatter: Add track management to timeline description

command-line-formatter: Add track management to timeline description

Instead of having it all handled by the tool, this way we can
set the restriction before clips are added to the timeline,
leading to better behavior in term of video images placement
in the scene.

Without that we would have the clips positioned before setting the
restriction caps which leads to weird behavior for the end users.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/227>