Fix "\" keyboard shortcut on German keyboard layouts

Summary: Ref T10252. This is similar to D16259, but makes KeyboardShortcutManager more relaxed about altKey when typing obscure characters.

Test Plan: Pressed Option + Shift + 7 on a German keyboard layout, saw Conphernece sidebar toggle.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10252

Differential Revision: https://secure.phabricator.com/D16260