This commit introduces a client-side table filter that supports regexes
and propositional logic to filter table rows.
A table can be filtered if it has the [filter-target] class and its
first row has <th> tags with a [data-filter] attribute specifying column
names.
The filter itself is a div with the [form] and [filter] classes, and a
[data-target] attribute pointing to the table to filter. The filter
contains a text <input> which is passed to filter_update() when the
filter expression is validated.
The client-side filter code runs the expression through a basic lexer
and parser, then matches the result for every row in the target table.
The [textContent] of each cell is used for string and regex matching.
