gst: Don't ref_sink() GstObject subclasses in instance_init/constructor

gst: Don't ref_sink() GstObject subclasses in instance_init/constructor

This is something bindings can't handle and it causes leaks. Instead
move the ref_sink() to the explicit, new() constructors.

This means that abstract classes, and anything that can have subclasses,
will have to do ref_sink() in their new() function now. Specifically
this affects GstClock and GstControlSource.

https://bugzilla.gnome.org/show_bug.cgi?id=743062