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.
