Bug 927862 - Implement AttentionWindow (WIP 2.1)

Conflicts:
	apps/system/index.html
	apps/system/js/activity_window.js
	apps/system/js/app_transition_controller.js
	apps/system/js/app_window.js
	apps/system/js/app_window_manager.js
	apps/system/js/attention_screen.js
	apps/system/js/browser_mixin.js
	apps/system/js/cards_view.js
	apps/system/js/layout_manager.js
	apps/system/js/orientation_manager.js
	apps/system/js/popup_manager.js
	apps/system/js/screen_manager.js
	apps/system/js/statusbar.js
	apps/system/js/utility_tray.js
	apps/system/js/visibility_manager.js
	apps/system/style/window.css
	apps/system/style/zindex.css
	apps/system/test/unit/activity_window_factory_test.js
	apps/system/test/unit/activity_window_test.js
	apps/system/test/unit/app_window_manager_test.js
	apps/system/test/unit/homescreen_window_test.js
	apps/system/test/unit/layout_manager_test.js
	apps/system/test/unit/mock_app_window.js
	apps/system/test/unit/mock_attention_screen.js
	apps/system/test/unit/screen_manager_test.js
	apps/system/test/unit/statusbar_test.js
	apps/system/test/unit/utility_tray_test.js
	apps/system/test/unit/visibility_manager_test.js
	test_apps/uitest/js/UI/window_open.js