Add a final state on dunning
For now, when a dunning is on the last level of the procedure it stays in the 'done' state until it is paid. But nothing remember the user that this is still a pending dunning and that the system will do nothing about it.
Indeed usually at this level, the procedure goes to court etc.
I propose to put the dunning in a final state which is different from the done. And to rename the done into waiting.