The integration tests aren't passing yet, but this adds the infrastructure
to support CURRENT_FILE and CURRENT_PROJECT lookups. Additional work remains
for ensuring that prefs for a specific file work properly and to improve
event dispatch to better support these new contexts

This was the starting point of a refactoring to better handle a larger, disjoint
set of path-based Scopes. I can see complexity brewing and I don't think it's
worth it for path-based Scopes. After this commit, I'm going to yank out all of
the path-based Scopes code and switch to simpler, project-based approach.

This fixes linting.enabled, for example.

* Changed `Editor.optionChange` event to send Brackets preference names
* Updated the doc comments for the editor options functions in Editor.js
* Fixed setValueAndSave to use the new behavior.