More complex way to add/update list item

[maraf]

Sometimes it's benefitial to build the item in several steps

<list:add name="people">
  <list:set key="name" value="Jon" />
  <list:set key="age" value="20" />
</list:add>
<list:add name="people">
  <list:set key="name" value="Jane" />
  <list:set key="age" value="24" />
</list:add>

Coordinate with #524 for the same experience with list:update