Use namespace in the registry and add array methods.
Actually setup a listener for disposable instances.
Add array methods to the list and implement has.
Rename `removeListener` to `off` to be more consistent.
Factor out recurring task of window state observing into separate module.
Factor out style-sheet service interaction APIs into separate module.
Implement page-mod without legacy APIs.
Don not delay test callback otherwise tests like `testMixedContext`
won't have a done callback in time.

Delay test actual tests that need to do delayed assertios.
Merge branch 'jep/disposable' into cleanup/page-mod
Merge remote-tracking branch 'upstream/master' into cleanup/page-mod
Merge branch 'cleanup/traits' into cleanup/page-mod

Conflicts:
	lib/sdk/util/registry.js
Implement page-worker without use of legacy APIs.
Remove PRIVATE_KEY that is no longer necessary.
Merge branch 'cleanup/page-mod' into cleanup/traits
Merge branch 'cleanup/page-worker' into cleanup/traits
Update page worker implementation such that it no longer uses symbiont.