Make a global (or project-based) setting for adding journal entries when one adds/removes a watcher.

Giving the code a quick look, it feels like this could be implemented by adding some logic to the "set_watcher" method in app/controllers/watchers_controller.rb to create a Journal entry.

I am currently implementing the functionality on top of the Redmine Watcher Group plugin http://www.redmine.org/plugins/redmine_watcher_groups (for which I also have a need, but this is unrelated to the current issue) with a patch object, but this feels like it would be more appropriate within core features.