keyboard: Track all key combos

For shortcuts that support more than one binding, we currently simply
ignore all but the first. This makes some sense as we only expose a
single binding in the UI anyway, however it means that any extra
bindings are not taken into account for conflict resolution. In order
to address this in the future, start tracking all bindings of an item.

https://bugzilla.gnome.org/show_bug.cgi?id=673078