Register deserializers, view providers, and commands in Atom constructor

Previously, these registrations were scattered all over the codebase and
required that `global.atom` be defined, which creating weird ordering
issues. Registering them all from the global makes the relationships
clearer and means we don’t have to assign a global to set up a fully-
functional environment.

src/workspace.coffee

@@ -26,8 +26,6 @@ Task = require './task'
 #
 module.exports =
 class Workspace extends Model
-  atom.deserializers.add(this)
-
   @deserialize: (state) ->
     return unless state?