This was complexifying the implementation for very little gain.
Each source type should ideally have its own API.
In that patch we make it so we do not have to subclass anything
but instead use GESAsset to pass information about how the pipeline
should look like.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/175>
We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
With two exceptions:
* ges_clip_create_track_elements_func
* ges_uri_clip_set_uri
which were never declared in headers and should always have been static.
Most of the time the user should not create GESTrackElements
himself, instead he should add a GESAsset to a layer, that will
result in a clip creation and the proper TrackElements to be
created and added to the tracks.
The case of effects and overlays is a bit different as the user should
create the TrackElement and add them to a clip.
