Summary:
Make `PhabricatorMenuView` more flexible, so callers can add items to the beginning/end/middle.
In particular, this allows event handlers to receive a $menu and call `addMenuItemToLabel('activity', ...)` or similar, for D4708.
Test Plan: Unit tests. Browsed site. Home page, Conpherence, and other pages with menus look correct.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Differential Revision: https://secure.phabricator.com/D4792
