This commit introduces the 'rainbowification' feature to the web client
(and to some affected addons).  This feature is essentially a way to
display a nice friendly message when some business event happens.  For
example, a encouraging message is displayed when the user clear her/his
inbox, or when a salesman closes a deal.

The mechanism currently only displays a 'rainbowman', but could be
extended later to add other kind of animation.

There are a few different ways to display a 'rainbowman':
- any JS code can simply trigger_up an event ('show_effect') with some
  options: for example
    this.trigger_up('show_effect', {
        type: 'rainbow_man',
        fadeout: 'no',
        message: $done,
        click_close: false,
    });
- an action returned by the server can have an 'effect' key, with some
  options
- the do_execute_action method accepts a 'effect' option.  This is
  useful when one wants to specify a rainbow on an action button in a
  form view

Joint work with: dbh <dbh@odoo.com>, ged <ged@odoo.com>