Make single key shortcuts optional

This patch converts MainWindow's keyboard shortcuts to be keybinding ("action") signal based, removes single key shortcuts by default, removes the gross hack that the old single key shortcut implementation required, adds a setting and preference for enabling single key shortcuts again, and updates both standard and single key shortcuts shortcuts in use (the latter now actually matches GMail's). Also updates the help, and converts the preference window to use libhandy and hence be responsive and adds some new bonus keyboard navigation shortcuts.

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