Subprojects simply consist of adding the GESProject
to the main project asset list. Then those are recursively
serialized in the main project in the <asset> not, when deserializing,
temporary files are created and those will be used in clips
as necessary
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.
