Enable display of tiddlers in a tank in policymgr

This is related to #113, providing a way to see if there are
any tiddlers in a tank you are considering deleting.

This could be extended later to:

* allow deleting tiddlers
* bulk managing tiddlers in some fashion
* dropping tiddlers into other tanks

But for now it just lists them.

There is a danger this will not work well for tanks that have
_many_ tiddlers.