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 aim is to expose the underlying cross-platform evalGlobal and
evalSandboxed.
Also adding the capability for a tiddlywiki.files file to specify a
prefix to be added to the text of a file.
We were unpacking plugin tiddlers in arbitrary order, and ensuring that
later plugins didn't overwrite shadow tiddlers from earlier plugins.
Now we'll allow plugins to specify a "pluginPriority" that determines
the load ordering. We also explicitly allow shadow tiddlers from later
plugins to overwrite shadow tiddlers from earlier plugins.
We're setting a base priority on the core plugin, since many plugins
will want to control their loading relative to it.
