[FIX] sale_timesheet: default so line for subtasks

When creating a task in a project generated from
an SO line, we want this to be linked to that
sale line too.
When creating a subtask, the sale line is forced
to be the one of the parent, in `create` method.
But we want to see it on the form view.

addons/sale_timesheet/models/project.py

@@ -56,7 +56,16 @@ class Project(models.Model):
 class ProjectTask(models.Model):
     _inherit = "project.task"
 
-    sale_line_id = fields.Many2one('sale.order.line', 'Sales Order Item', domain="[('is_service', '=', True), ('order_partner_id', '=', partner_id)]")
+    @api.model
+    def _default_sale_line_id(self):
+        sale_line_id = False
+        if self._context.get('default_parent_id'):
+            sale_line_id = self.env['project.task'].browse(self._context['default_parent_id']).sale_line_id.id
+        if not sale_line_id and self._context.get('default_project_id'):
+            sale_line_id = self.env['project.project'].browse(self._context['default_project_id']).sale_line_id.id
+        return sale_line_id
+
+    sale_line_id = fields.Many2one('sale.order.line', 'Sales Order Item', default=_default_sale_line_id, domain="[('is_service', '=', True), ('order_partner_id', '=', partner_id)]")
 
     @api.model
     def create(self, values):