This is a hacky way of having macro definitions that behave as if they
are global to the wiki.
The new tabs macro itself is cool, though: neatly encapsulates most of
the gubbins around a tab control, and the rendering is quite easy to
style.
Belatedly realised that the design would be clearer without these two
separate concepts being conflated into a single widget.
As a result of this change, any other widget or template that generates
transclude widgets has needed adjustment.
Templates are the low-level bits and pieces that allow TW5 to generate
HTML and CSS renderings of tiddlers. The UI folder contains the
user-visible UI of TW5
