Convert DataGUI into a bona fide component

[ashmaroli]

In order to minimize repeated logic.
(However, there's still some logic repeated in the three components that consume DataGUI: DataFileNew, DataFileEdit and Configuration. But that seems to be necessary at the moment.)

Currently on master, DataGUI is just Metafields rendered with prop dataview: true and the inputs for editing data file path being additionally implemented for both DataFileNew and DataFileEdit.

Another internal change is to use input elements' ref value on save instead of setting the parent component's state and then reading from that value.