[FIX] purchase: do not pass uom in product_id_change when changing of product

To allow the on change to set the product default unit of measure when changing of product, and only in this case (not when changing price or quantity)
[ADD] actions documentation
[FIX] actions: match the behavior of multi actions with its help text

also fix the corresponding text and add explicit sequence number because I
don't understand what the bloody hell it does without that, except that it's
not the right thing. At all.
[IMP] new-api-ify 'trigger' server action
[I18N] Update translations from Launchpad 8.0 branches
[IMP] fields: make attribute 'default' callable
[IMP] fields: set the default value to the closest field.default or _defaults

This solves a subtle issue: in the following case, the class Bar should
override the default value set by Foo.  But in practice it was not working,
because _defaults is looked up before field.default.

    class Foo(models.Model):
        _name = 'foo'
        _columns = {
            'foo': fields.char('Foo'),
        }
        _defaults = {
            'foo': "Foo",
        }

    class Bar(models.Model):
        _inherit = 'foo'
        foo = fields.Char(default="Bar")

The change makes field.default and the model's _defaults consistent with each
other.