* Bug in plugin Ordering
Comparing plugin A without a `plugin-priority` field to plugin B with a `plugin-priority= 1` results in the plugins being ordered B, A when it seems like they should be ordered A, B.
* Switch to setting a plugin-priority on the built in themes to allow overriding parts of them.
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