web: Add ability to have bar graphs in table rows

<div class="bar-row" graph="name" value="10/30"> becomes a row of
a graph. $cockpit.BarRow('name').attr('value', '10/30') creates
a similar row.

See controls.js for more details on how to use this.

Add a dependency on attrchange.js jQuery plugin, which is really
useful and saved me nearly as much code in controls.js.