dep-ui.js: Use `input' events for updating deps. Factor `input_...'.

Somehow I missed the `input' event on all formish widgets, which is, of
course, exactly what I wanted all along.  Use this for everything,
rather than a mishmash of other widget-class-specific events.  This
means, in turn, that `input_field' and `input_radio' are almost exactly
the same function now, so factor out the common machinery.