[UI] Improve "Continue only if"

[mutmut]

Content moved to new feature request (see below)

I added the blocks in the title because I just noticed that you can never delete block 1 even if it is empty:

I’d like to have the same behavior, what about you?

EDIT: this looks like the second part of @StephaneB’s request here:
[Scènes] Pouvoir insérer un bloc de scène (plutôt qu'un ajout en fin)