[IMP] Add action for choosing a variant as the price reference

- This can be very handy for the website if we want to use not the cheapest variant as the default
  displayed price. It also reorders the attribute values such that when opening a product on the
  website, it immediately shows the reference variant.

product_variant_sale_price/__manifest__.py

@@ -12,6 +12,9 @@
     "license": "AGPL-3",
     "installable": True,
     "depends": ["account", "sale"],
-    "data": ["views/product_views.xml"],
+    "data": [
+        "data/server_actions.xml",
+        "views/product_views.xml"
+    ],
     "post_init_hook": "set_sale_price_on_variant",
 }

product_variant_sale_price/data/server_actions.xml

@@ -0,0 +1,17 @@
+<odoo>
+    <record model="ir.actions.server" id="set_price_reference">
+        <field name="name">Set price reference</field>
+        <field name="model_id" ref="model_product_product"/>
+        <field name="state">code</field>
+        <field name="binding_model_id" ref="model_product_product"/>
+        <field name="binding_type">action</field>
+        <field name="code">
+if len(records) > len(records.mapped("product_tmpl_id")):
+    raise UserError("You can only set one variant as the price reference of a product.")
+for r in records.with_context(skip_update_fix_price=True):
+    r.product_tmpl_id.write({"list_price": r.fix_price})
+    new_seq = min(r.product_tmpl_id.attribute_line_ids.mapped("value_ids.sequence") or [r.fix_price])
+    r.mapped("product_template_variant_value_ids.product_attribute_value_id").write({"sequence": new_seq - 1})
+        </field>
+    </record>
+</odoo>