effect: Add support for time effects

effect: Add support for time effects

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>