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/pane-axis.coffee

@@ -4,8 +4,6 @@ Model = require './model'
 
 module.exports =
 class PaneAxis extends Model
-  atom.deserializers.add(this)
-
   parent: null
   container: null
   orientation: null