keyboard: Consider additional bindings in uniqueness checks

We now have everything in place to extend the uniqueness check to
consider all bindings of an item rather than just the first one.
With this it is finally possible to set Alt+Tab as binding for
"Switch windows" without keeping the hidden Alt+Tab binding of the
"Switch applications" shortcut ...

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