* Add a type=page to the top-level ViewPage instances
* Rename viewTabVisibleCallback and viewTabHiddenCallback to view*Callback and call each when selectedness or visiblity changes
* Ensure active view/pages are always properly initialized during page load and category switching
* Add a test to verify no mutations happen when tabs change while firefox view is inactive
* Fix tests to better account for loading and readiness sequence when activating firefox view
Differential Revision: https://phabricator.services.mozilla.com/D193744