Improve livedata edit mode's consistency

With livedata's edit mode, there are a few inconsistencies between editing existing rows/cards and creating new ones:

• save is transactional for new rows, with manual validation saving all the cells, and automatic for each cell for existing ones
• the displayers appear broken for unsaved cells of new rows (fixed only after validating the creation)

Ideally, these two usecases should have similar behaviors.