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.
