Getting rid of the old widget mechanism files finally gives us a payoff
for all the refactoring. Still a bit of tidying up to do, and we need
to re-introduce the animation mechanisms.
To start with, we move the current stylesheets into a theme plugin
called "Snow White". Wikis have to specify at least one theme in their
`tiddlywiki.info` file. Next we'll add a mechanism for switching
between loaded themes
