Add a central notifier for keeping views in sync

[davvid]

Right now the classic and repo status views can get out of sync since they each have their own copy of the model and do not communicate with each other.

cola needs a central cola.notifier module to handle cross-object messages. Much of the tightly coupled operations (such as clicking on a file in the repo status view) could be decoupled by having the selection and other events going through the notifier.

Each view could then simply pickup messages from the notifier and react accordingly. This should allow us to, for instance, click on a filename in the classic or status views and have the diff show up in the diff view.

Full-blown updates should also go through the notifier so that refreshing one view refreshes them all.

[davvid]

We now have cola.notifier() and it's used by the new views.status widget. All we need now is a central command factory to watch for the broadcasted signals and issue appropriate cola commands.

[davvid]

This is probably about 75% done. The latest work is in 'next'.

[davvid]

cola: Integrate the SelectionModel

Selection is now handled by a static SelectionModel instance.
This allows individual tools to modify the selection and have
the selection reflected in the corresponding menus and actions.

Closed by d1f7b7c

Signed-off-by: David Aguilar davvid@gmail.com