Implement addon/window API.
Simplify hidden-frame implementation by using
addon/window instead of platform specific hidden one.
Merge branch 'master' into bug/addon-window@820953

Conflicts:
	lib/sdk/addon/runner.js
Merge pull request #698 from Gozala/bug/addon-window@820953

Bug 820953 - Expose DOM window to the add-on r=@Mossop