Accessories: an extension architecture for Bazaar Explorer

[Ian Clatworthy]

I'm highly excited to announce the availability of accessories, the cool
new way to extend Bazaar Explorer. Accessories enable several things:

* the ability to enable/disable optional out-of-the-box features
* the ability for plugins to add features (and for these to be
  optionally disabled if you don't want them)
* a "wallet" for managing your personal extensions
* a "hat" for path-aware extension activation.

To test out this feature, grab rev 392 or later and experiment with the
Accessories dialog under the File menu. (This replaces the old "Switch
hat" menu btw.) It's also worth grabbing rev 272 of bzr-fastimport to
see how a plugin can add goodies to the Tools menu.

Currently, an accessory may provide bookmarks, custom editor settings,
logos (additional icons) and tools. I plan to extend that in the future
to include toolbars, logical kinds (e.g. being able to mark a branch as
a 'review' branch or 'packaging' branch), context menus and more.

I'm really proud of this feature and I hope you enjoy using it. For
those of you that tried Explorer 0.1 and were curious about why I
included a "hats" feature, I hope it now makes a lot more sense. In
particular, these latest changes bring Explorer one step closer to being
the "IDE-without-an-editor" vision I've had for it since day 0.

Ian C.