This actually should be the “payments and receivables” section, however
this is quite a mouthful; a “receivable” is a payment made **to** you,
therefore “payments” is ok.
In fact, there is still no receivables in there, as they should be in
a separate relation, to constraint them to invoices instead of expenses.
It will be done in a separate commit.
Since this section will be, in a sense, sort of simplified accounting,
i needed to introduce the “payment account” concept. There is no way,
yet, for users to add them, because i have to revamp the “tax details”
section, but this commit started to grow too big already.
The same reasoning for the attachment payment slips as PDF to payment:
something i have to add, but not yet in this commit.
