Label formatting: add ability to add references to label block lines

[arbolitoloco]

Feature suggested by @mickley:

it would be nice if there was a different class name included for each label line (e.g., line1), rather than just fieldblock, so that each line could be styled individually with some ease.

I'm thinking this would be in the form of ids and not classes. Would need to be worked in the format editor, as well as the visual interface editor.