timeline: fix adding track when layers contains clips

timeline: fix adding track when layers contains clips

Made sure that adding a new track only uses select-tracks-for-object for
core children to determine whether a track elements should be added to the
new track or not, and *not* any other track. In particular, there should
be *no* change in the existing tracks of the timeline when adding another
track. Moreover, a new track should not invoke the creation of track
elements for other tracks.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/160>