Add support for data table search

[okennedy]

A common use case for data tables is text-search. In principle, the browser's normal "find" feature should be ideal for this, but we're not materializing the full DOM of a table. That means that unfortunately we need to provide our own search.

Ideally, this should be triggered by a search box in the upper-right, and/or by overriding Ctrl-/Cmd-F in spreadsheet mode.