Thread: Don't kill all threads in spawned process.

No need to do so as there shouldn't be any required activity going on in
the threads.
PhotoManager: Check rotation fail in correct spot.

The checking for whether the photo rotation was successful was in the
wrong callback.
PhotoManager: Lower the priority of photo caching.

Lower the priority of the process that runs the photo caching and photo
rotation.
Page: Move lifetime countdown to the Page View.
PageManager: Pause overlay timeout on page spawn.

When a Page creates another Page, pause any timeouts on the underlying
Page.