This likely broke in 3.x when Ardour migrated to context dependent
shortcuts (rather than GTK managed menu shortcuts).
While there is Gtkmm2ext::Application::hide, there is currently no
bindable action and hence no way to add a dedicated
shortcut. Furthermore, since this is a special NSApplication menu,
updating the menu-item when the action is bound is not trivial.
gtk_application_hide() is only implemented for macOS and
there is likely a better use for "H" shortcut anyway.
ac53f7e95a