keyboard: Refactor finding of conflicting items

When comparing keys for uniqueness, we currently apply various tests to
check whether shortcuts are different, until we decide that we found a
conflict if none of the tests passed. That approach is a bit weird for
shortcuts that have a reverse item - when comparing a binding to two
different shortcuts, it should always be different from at least one of
them, so there should never be a conflict for any reversible shortcuts.
The reason it does work anyway is that reverse items usually only differ
in modifiers, which is_shortcut_different() currently doesn't consider at
all. We are about to change that however, so refactor the code to set the
conflicting item as soon as we find a match rather than as fall-through.

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