Allow only one global top menu to be open at a time

Summary: Fixes T3715. Makes "visible" global instead of per-menu, so all the menus share a visible state.

Test Plan: For menus A and B, clicked "A, A", "A, B, A", "A, B, B", "A, B, B, A", etc. Couldn't figure out a way to break it.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T3715

Differential Revision: https://secure.phabricator.com/D6745