Clear data store button

[frabcus]

Nicola asked for this.

I'm not completely sure either way.

[zarino]

I have a feeling, but no proof, that people will expect this button to be on the Table view.

[frabcus]

@JacobJWalker in #98 certainly seemed to expect it on the "View in a table" view, and/or to be able to use "Query with SQL" to type it in.

In terms of the interface, it would fit most naturally in the code in the browser tool. It's not generally the case that you need to be able to clear datasets for other importing tools.