Add a Quicksand-aware page-level container to diffs

Summary:
Ref T12616. Ref T8047. Ref T11093. We currently have several bugs where diff state sticks across Quicksand pages.

Add a new top-level object to handle this, with sleep() and wake() methods. In sleep(), future changes will remove/deacivate all the reticles/editors/etc.

See T12616 for high-level discussion of plans here.

This general idea is likely to become more formal eventually (e.g. for "sheets" or whatever we call them, in T10469) but I think this is probably a reasonable place to draw a line for now.

Test Plan:

• Added some logging to sleep(), wake() and construct().
• Viewed changes in Differential.
• With Quicksand on, browsed around; saw state change logs fire properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616, T11093, T8047

Differential Revision: https://secure.phabricator.com/D17840