[FIX] base, website_theme_install: review todo action order

Since https://github.com/odoo/odoo/commit/d1fc62df0b850c56360c1e5391c69eb807d0fe46:
When several ir.actions.todo are created, only the last one is
considered and the first are marked as done. Thus, there are never
multiple "open" todos at the same time (while before this, only the
first todo was done and the other ones were done on subsequent module
installations, which makes no sense).

The new system however broke stuff. Indeed, when installing the
eCommerce, we expect to land on the "Choose a Theme" page (if website
was not already installed). This was not the case anymore, as the
last todo in that installation is "Redirect to /shop". To fix the
problem, the system is changed to not consider the "last todo" but
"the todo with highest priority, or the last one if same priorities".
By making the "Choose a theme" todo high priority, this solves the
problem.

A better system would be to make todos depend on each other... but
that would be a task for master.

addons/website_theme_install/views/views.xml

@@ -86,6 +86,7 @@ action = {
     </record>
     <record id="theme_install_todo" model="ir.actions.todo">
         <field name="action_id" ref="theme_install_todo_action"/>
+        <field name="sequence">0</field>
     </record>
 
     <!-- Add link to theme install in default customize modal -->

odoo/addons/base/ir/ir_actions.py

@@ -602,18 +602,19 @@ class IrActionsTodo(models.Model):
     def create(self, vals):
         todo = super(IrActionsTodo, self).create(vals)
         if todo.state == "open":
-            todo.ensure_one_open_todo()
+            self.ensure_one_open_todo()
         return todo
 
+    @api.multi
     def write(self, vals):
         res = super(IrActionsTodo, self).write(vals)
         if vals.get('state', '') == 'open':
-            for todo in self:
-                todo.ensure_one_open_todo()
+            self.ensure_one_open_todo()
         return res
 
+    @api.model
     def ensure_one_open_todo(self):
-        open_todo = self.search(['&', ('state', '=', 'open'), ('id', '!=', self.id)])
+        open_todo = self.search([('state', '=', 'open')], order='sequence asc, id desc', offset=1)
         if open_todo:
             open_todo.write({'state': 'done'})