bus, clock: make sure these never have a floating ref

bus, clock: make sure these never have a floating ref

Clear the initial floating ref in the init function for
busses and clocks. These objects can be set on multiple
elements, so there's no clear parent-child relationship
here. Ideally we'd just not make them derive from
GInitiallyUnowned at all, but since we want to keep
using GstObject features for debugging, we'll just do
it like this.

This should also fix some problems with bindings, which
seem to get confused when they get floating refs from
non-constructor functions (or functions annotated to
have a 'transfer full' return type). This works now:

from gi.repository import GObject, Gst

GObject.threads_init()
Gst.init(None)

pipeline=Gst.Pipeline()
bus = pipeline.get_bus()
pipeline.set_state(Gst.State.NULL)
del pipeline;

https://bugzilla.gnome.org/show_bug.cgi?id=679286
https://bugzilla.gnome.org/show_bug.cgi?id=657202