Can't resize empty table

A newly-created Table.input can't be resized.

When attempting to resize a component containing Table.input (by its widget, not the visualization), an error message is logged: "Tried to set metadata on arg placeholder. This is not implemented yet!" It works if the component code is Table.input [], but this is not the default when creating a table component with the CB.

The cause is a technical limitation: We attach the table widget to the argument of Table.input, which may be code or a placeholder (if no argument is provided in source code, and the default value [] is used). When the value is a placeholder, we have no source code to associate the size metadata with. I think we should fix this by attaching the table widget to the whole Table.input expression.