timeline: create auto-transitions during track-element-added

timeline: create auto-transitions during track-element-added

Any time a track element is added to a track, we need to check whether
we need to create a new corresponding auto-transition. This simply moves
the code from ges-clip.c to ges-timeline.c, where it is more appropriate.

Moreover, it technically opens the possibility for creating
auto-transitions for track elements in the timeline that have no
corresponding clip.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/169>