Bug 961800 - Implement Child Window Factory.

Conflicts:
	apps/system/js/app_transition_controller.js
	apps/system/js/app_window.js
	apps/system/js/app_window_manager.js
	apps/system/js/cards_view.js
	apps/system/style/window.css
Merge pull request #17514 from alivedise/bugzilla/961800_master_merging/child-window-factory

Bug 961800 - Implement Child Window Factory. r=etienne