Fixed problem with plugin precedence

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.
Added basic support for drag and drop of tiddlers

Links are now draggable. Dragging outside the browser will drop the
wiki text of the tiddler. Dragging to another TW5 browser window will
drop the entire tiddler, including fields.