Abstract logic to find unique slug of a List

[wernerkrauss]

As discussed on slack, it'd be nice to abstract the logic to find a unique slug in a given SS_List. This could be "all elements of a parent page" or e.g. "all elemental blocks of a given page or area"

[wernerkrauss]

See https://slackarchive.silverstripe.org/slack-archive/channel/content-blocks/?message=279258&start=730 for the slack conversation