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.
To avoid compiler warning when using const string to create a new
GESUriClip as string is not modified and only passed to functions which
take a const string.
https://bugzilla.gnome.org/show_bug.cgi?id=766523
