This is a pure refactor in preparation for letting users configure multiple
alternate bindings for a single command. Every Binding still has exactly one
key, so nothing changes visibly: the cheatsheet, the on-screen options bar,
and the keybindings menu all render identically.
When a Binding ends up with multiple keys, the on-screen options bar will
show only the first (to avoid clutter); the cheatsheet will show all of them (in
a later commit). For now both paths take Key[0].
MenuItem.Key is changed in the same way, it also has a slice of keys now.
In this commit we keep the name `Key` in Binding, KeybindingOpts and MenuItem,
instead of renaming them to `Keys` right away, in order to keep the diff a bit
more readable. We'll do the rename separately in the next commit.
Constructing a menu item key from a literal character requires
gocui.NewKeyRune('r'), which is a bit noisy. Add a private menuKey helper in
both the controllers and helpers packages so the common case in either reads as
menuKey('r'). Duplicating the one-liner is cheaper than a cross-package import
dependency and avoids forcing every controller file to qualify the call.
The reason for doing this now is that we are going to change MenuItem.Key to a
slice of keys later in the branch, which means we'd have to add `[]gocui.Key{`
at each call site, making them even more noisy. With the menuKey helper we can
just change its signature and leave all clients unchanged.
