Ease defining callback inputs and outputs

[forman]

Instead of channel links we should use special id format such as "@<link>".

Advantage is that we can stick to the syntax Input(id, property), e.g.,

Input("@app", "selectedDatasetId")

instead of

Input(source="app", property="selectedDatasetId")

Other advantage: less redundance, less verbose user code and transport JSON, one keyword argument less.