Tree-style location selector for Bazaar Explorer

[Ian Clatworthy]

I've just landed a really cool patch from Gordon Tyler which adds a 
tree-style location selector to Bazaar Explorer. To try this out:

1. Grab rev 464 or later of trunk.
2. Open the Preferences/Options dialog.
3. Change the Location Selector Style to "tree".
4. Restart Bazaar Explorer as directed.

This is a feature I've wanted for a very long time. Thanks Gordon!

Ian C.