Move "hover over an inline to see the affected lines" code to the new class tree

Summary:
Fixes T8047. Ref T12616. Fixes T9270. This moves the "hover" part of the hover/drag behavior to the new code, leaving the "drag" part for a followup change.

The new hover UI behaves properly with Quicksand (T8047) and the filetree (T9270).

Test Plan: Hovered over inlines, saw lines select properly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616, T9270, T8047

Differential Revision: https://secure.phabricator.com/D17919