We wrap the [reset] anchor tag in a button so that we can set 'disabled'
attribute on it. We change the styles to hide the [reset] button and the
pencil icon when the widget is disabled.
We also need to call `e.preventDefault()` in the event handler since now
the anchor tag behaves as a button.
Previously, we tried to read the value from page_params, which was just
a hack to make the calling code look cleaner. We now remove that hack
and thus, our dependency on page_params existing. Now, if the caller
does not specify a default value, we'll use the null-value.
This also creates a new init() function to cleanly wrap the code that
makes changes to the opts passed to the widget.
