This modifies `get_active_binding` to treat bindsym --to-codes
separately: per each keysym `i` in `binding->keys`, look through the list of
matching keycodes in `binding->translations[i]`.
Another solution is to take the cartesian product of all syms and make a
binding per each product. This makes retranslation more difficult though
because the dups from the old layout have to be cleared out before
translating to the new `xkb_layout`. Whether by retaining a parent
binding, searching through all bindings for matching `binding->command`,
or some other ref count structure, that would introduce more complexity
than modifying get_active binding to account for dups. Notice this
requires no changes to the existing retranslation logic.