controllers: add new proxy control binding

controllers: add new proxy control binding

Allows proxying the control interface from one property on one GstObject
to another property (of the same type) in another GstObject.

E.g. in a parent-child relationship, one may need to
gst_object_sync_values() on the child and have a binding (set elsewhere)
on the parent update the value.

Note: that this doesn't solve GObject property forwarding and must be
taken care of by the implementation manually or using GBinding.

https://bugzilla.gnome.org/show_bug.cgi?id=774657