Renaming Content Blocks in Set

[mattmultiplied]

We've had a client request the option to assign custom labels to replicator sets. We have implemented the recommended solution from Idea #249 by using replicator_preview: false although it doesn't make it obvious what the context of each block is.

It would be good to have an option to assign custom block labels which overrides the default block label so it's a little clearer what each block when you have quite a few of them on a page. Mocked up how I'd see it looking down below.