[IMP+REF] contract: Allow to set recurrency at header level

Big refactoring for allowing to define recurrency at header level for simplifying
the use of the module for most of the cases where you don't need different
recurrency at line level.

contract/models/contract.py

@@ -20,6 +20,7 @@ class ContractContract(models.Model):
         "mail.thread",
         "mail.activity.mixin",
         "contract.abstract.contract",
+        "contract.recurrency.mixin",
     ]
 
     active = fields.Boolean(default=True,)
@@ -43,6 +44,16 @@ class ContractContract(models.Model):
         inverse_name="contract_id",
         copy=True,
     )
+    # Trick for being able to have 2 different views for the same o2m
+    # We need this as one2many widget doesn't allow to define in the view
+    # the same field 2 times with different views. 2 views are needed because
+    # one of them must be editable inline and the other not, which can't be
+    # parametrized through attrs.
+    contract_line_fixed_ids = fields.One2many(
+        string="Contract lines (fixed)",
+        comodel_name="contract.line",
+        inverse_name="contract_id",
+    )
 
     user_id = fields.Many2one(
         comodel_name="res.users",
@@ -53,12 +64,7 @@ class ContractContract(models.Model):
     create_invoice_visibility = fields.Boolean(
         compute="_compute_create_invoice_visibility"
     )
-    recurring_next_date = fields.Date(
-        compute="_compute_recurring_next_date",
-        string="Date of Next Invoice",
-        store=True,
-    )
-    date_end = fields.Date(compute="_compute_date_end", string="Date End", store=True)
+    date_end = fields.Date(compute="_compute_date_end", store=True, readonly=False)
     payment_term_id = fields.Many2one(
         comodel_name="account.payment.term", string="Payment Terms", index=True
     )
@@ -122,6 +128,8 @@ class ContractContract(models.Model):
             .search([("contract_line_id", "in", self.contract_line_ids.ids,)])
             .mapped("move_id")
         )
+        # we are forced to always search for this for not losing possible <=v11
+        # generated invoices
         invoices |= self.env["account.move"].search([("old_contract_id", "=", self.id)])
         return invoices
 
@@ -198,10 +206,15 @@ class ContractContract(models.Model):
                     and (not l.display_type or l.is_recurring_note)
                 )
             ).mapped("recurring_next_date")
-            if recurring_next_date:
-                contract.recurring_next_date = min(recurring_next_date)
+            # we give priority to computation from date_start if modified
+            if (
+                contract._origin
+                and contract._origin.date_start != contract.date_start
+                or not recurring_next_date
+            ):
+                super(ContractContract, contract)._compute_recurring_next_date()
             else:
-                contract.recurring_next_date = False
+                contract.recurring_next_date = min(recurring_next_date)
 
     @api.depends("contract_line_ids.create_invoice_visibility")
     def _compute_create_invoice_visibility(self):
@@ -276,7 +289,6 @@ class ContractContract(models.Model):
             vals["date_start"] = fields.Date.context_today(contract_line)
             vals["recurring_next_date"] = fields.Date.context_today(contract_line)
             new_lines += contract_line_model.new(vals)
-        new_lines._onchange_date_start()
         new_lines._onchange_is_auto_renew()
         return new_lines