Autocomplete suggestions from Data module table

We're using the Feeds and Data modules to speedily import some basic data. Each set of data is used for a variety of purposes. In one case, we'd like to suggest a value based on certain data while also allowing arbitrary values.

Although this could be considered an edge-case (#564648: Data from external field) and I could've whipped up some custom PHP for the job, this feature will no doubt be useful for us in future and possibly others as well.

This patch adds a widget for selecting Data table fields for autocomplete suggestions.