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.
