Only set the style of the button widget if the style attribute is specified
Add 'default' attribute to button widget

This allows us to do things like tab controls properly.
Update macrocall widget to allow type of input and output to be specified

Means that you can invoke macros without parsing them as wikitext.
Fix up missing variable values

We were crashing if a variable was inadvertantly set to undefined
Style cleaning for tabs
Introduce a PageMacros tiddler containing a tabs macro

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.