Move keyboard focus reticle code to Differential

Summary:
Ref T12634. Using keyboard shortcuts in Differential currently relies on focus behavior in KeyboardShortcutManager.

This possibly made sense long ago, but no longer does, and leads to a whole slew of bugs where the reticle doesn't interact properly with anything else.

Move it to Differential so it can be made reasonably aware of edit operations, Quicksand navigation, etc.

This just moves the code; future diffs will actually fix bugs.

Test Plan: Used "n", "j", etc., saw the same behavior as before.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12634

Differential Revision: https://secure.phabricator.com/D17899