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.
This means we need to properly track the layer a clip was in. We now
keep track of the various signal IDs in a dedicated structure and
keep a ref on the layer an object is in.
http://phabricator.freedesktop.org/T88
