Use a slice of keys for each binding

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.