Allow quick prepopulation of token value on field mapping edit page

Tokens often follow a pattern in which only a couple characters change. The exercise of clicking the field, navigating to the appropriate token and clicking it (or typing it in), and then saving is often too many steps. This would provide a dropdown that allows selecting field values currently in use on other fields. Upon click, a jQuery event handler would copy the value of the selected option into the text field and set focus to it.

Granted, I'm not totally convinced of this solution; clicking a dropdown isn't much better. So I might not implement it, but figured I'd open the issue to, if nothing else, spark conversation.