protocol reference: replace "category filter" with a dropdown list, remove "identifier" and "description" filters

This replaces the "category filter" textbox with a dropdown list that
offers a choice between viewing a single category or all at once
(the default).

It also removes the "identifier" and "description" filters, because they
were too slow to be useful. Until we find a way to speed this up, the
browser's built-in search function is a much better way to find a specific
control.