We re-use some of the existing syncer mechanism. It was already keeping
track of changes to tiddlers in the store when working with a tiddler
syncadaptor. Now it also tracks changes when there is no syncadaptor,
allowing us to provide a warning if there are unsaved changes.
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
The refactored plugin is `tiddlyweb2` for the moment. The idea is to be
able to use the same syncer with a different adaptor for syncing
changes to the local file system.
