Remove obsolete stylesheet helper
Refactor the theme switcher to be a generic plugin switcher
Unlinkify the current theme name

Having it link to the underlying plugin tiddler isn’t very useful
Update control panel text
First pass at language plugins for internationalisation

Still quite a few details to work out, but this shows the basic idea of
re-using the theme mechanism to handle language plugins.

Comments and questions welcome.
Rejig the tabs macro to wikify captions

This will allow us to translate tab captions