Instead of chaining the adding of the scopes at the caller, scopes are now added in the order the addScope was called. For each scopeOrder, a shadowScopeOrder is maintained which tracks the scope load status. If the scope was specified explicit "before" location and the "before" scope failed to load, it will be added to the first successfully loaded scope after "before". "default" scope is always loaded for the context and it's always resolved.

Deficient $.when is replaced with new Async.waitForAll (which would call always only when all promises are either resolved or rejected).

cc: @dangoor -- this is working, but not mergeable fix for #6660. I'll create task list to complete it later today or tomorrow. Please let me know what you think.

TODO:

• load scopes if "before" scope has failed to load
• do not require the caller to chain promises to add scopes in order
• always create "user" scope, even if the one that has been requested has failed
• warn user on corrupt brackets.json ("user" scope) on start-up and open it for editing
• make sure preferences are loaded before any extension starts to load
• comment/document the implementation
• review test failures
• refactor the tests to catch async (e.g. load failures)