Allow PhabricatorMenuView to have items inserted in the middle

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