Blueprints: fix Block naming inconsistency

[mrflix]

Description

Currently in blueprints, every field is named by its label. Except blocks. Blocks are named by their title and the label value is used as a subtitle (TIL #4194). This leads to developer mistakes when naming custom blocks, because one would expect that the label value would also name blocks.

Expected behavior
I'd expect to be able to use label to name a custom block. For example by falling back to use label when a label and no title are defined.

Your setup

Kirby Version
4.0.2