play: Missing g_object_notify signals

Originally 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. GTK.

This also introduces API incompatibility with all apps that were using g_object_bind_property with player props and thus depend on them.