play: Missing g_object_notify signals
GstPlayer library had used
g_object_notify_by_pspec to notify about changed values on its properties. New
GstPlay lib unfortunately lost this ability along the way, which makes is harder to use with toolkits that allow binding GObject properties e.g.
This also introduces API incompatibility with all apps that were using
g_object_bind_property with player props and thus depend on them.