transcriberbin: fix property proxying
As part of !1546 (merged) the element started implementing the GstChildProxy interface in order to expose properties on its sink pads, but the implementation was incorrect and broke proxying to children elements.
In addition, an intermediary bin was introduced with no name, making it hard to set the properties of the inner elements through the child proxy interface, it is now named according to the name of the pad it corresponds to.
Finally, the default transcriber is back to being named "transcriber".