And deprecate all GESTrackElement constructors, but the GESEffect one.
Those should **never** be created by users and should become internal
in the future.
Stop having docstring for the constructors that were internal.
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.
