account_payment_promissory_note occludes due date of invoice_date_due

[risto42]

At least in 16.0, the following warning comes up numerous times:

WARNING <db> odoo.addons.base.models.ir_model: Two fields (invoice_date_due, date_due) of account.payment() have the same label: Due Date. [Modules: None and account_payment_promissory_note] 

robably should let the msgid be "Promissory Note Due Date" with the msgstr as "Due Date" if necessary to avoid stepping on toes of other modules, while keeping the noise out of the log file.

[pedrobaeza]

Please propose a PR changing the string.

[risto42]

Well, I'm not actually using this module as of yet, it's just muddying the waters with the warnings.

After looking at it, it appears that the module stopped in the middle of a migration to a mixin where two fields were doubled as well. So I put the fields into the mixin, kept the default label name and updated the translations.

Attached is the quick patch for someone who is active on this module to decide whether indeed it's worthwhile even to keep the mixin... as I don't see any readily available design rationale justifying it.
AP-731.zip