Allow the user to register a child property of a base effect as a time
property. This can be used by GES to correctly calculate the
duration-limit of a clip when it has time effects on it. The existing
ges_effect_class_register_rate_property is now used to automatically
register such time effects for rate effects.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/177>
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.
