Make editable provider cell in CVR sources table render as a ComboBox

[HEdingfield]

Per the discussion in #462.

@hedingfield

If you double click a cell to edit provider in the table do we care enough to try to make that an embedded dropdown?

@tarheel

It would be weird if it were an editable text box. If making it a dropdown is a pain, I would just disable editing entirely. The workaround (delete the source and add a new one) is such a small burden for the use

Here's a post on how to disable a single cell in an otherwise editable row.

And here are some posts (1, 2, 3) on how to make it appear as a ComboBox.