This make SequenceWidget and MappingWidget behave more consistent.
And: it's already stated in the docstring of Widget: "the default
mapping and sequence widgets are in this category ['structural']".
And: "Renders a sequence into a set of fields."
Plus: we get rid of the invalid LABEL element pointing to the widget's
DIV -- which is not valid HTML.
Instead of using custom (non-valid) attributes (e.g. prototype) and
a (non-valid) SPAN inside a sequence's UL to hold min/max variables,
we use HTML5 data-* attributes now.
For easy access to them (and for the hotness' sake) upgrade to
jQuery 1.5.2 and a more recent jQuery UI.