We no longer use the `<$link>` widget for external links. Instead the
parser generates `<a>` elements. This makes things simpler, but does
mean that the `target=_blank` behaviour is baked into the parser.
Probably we should introduce a new `<$extlink>` widget that generates
an `<a>` element with a configurable `target` attribute
This arrangement takes better advantage of the similarities between the
now deleted widget renderer and the element renderer. It also obviates
the need for wrapper elements around every widget.
