title: Value of custom field in sale.config.settings is not displayed in sales settings view
body: <p>I try to add a new configuration field for sales configuration settings by inheriting the standard sales settings view/model using a custom module. </p>
  
  <p>Creating the field in the model works fine and a value entered in the view is also successfully written to the database. 
  <strong>However, if I reload the view for sales configuration, the previously stored value is back to 0.00 (while still being ok in the table!).</strong></p>
  
  <p>Struggling with this for days now, found related posts by research (odoo site and stackoverflow) which all unfortunately did not work for me. </p>
  
  <p>This is the view definition XML file:</p>
  
  <pre><code>&lt;openerp&gt;
      &lt;data&gt;
          &lt;!-- SALE CONFIG FORM VIEW Section --&gt;
          &lt;record model="ir.ui.view" id="view_custom_sale_config_form_inherited"&gt;
              &lt;field name="name"&gt;sale settings&lt;/field&gt;
              &lt;field name="model"&gt;sale.config.settings&lt;/field&gt;
              &lt;field name="type"&gt;form&lt;/field&gt;
              &lt;field name="inherit_id" ref="sale.view_sales_config" /&gt;
              &lt;field name="arch" type="xml"&gt;
                  &lt;data&gt;
                      &lt;xpath expr="//div[@name='Sale Features']/div" position="inside"&gt;
                          &lt;div name="threshold_price_mgr_sig"&gt;
                              &lt;label for="threshold_price_mgr_sig"/&gt;
                              &lt;field name="threshold_price_mgr_sig" class="oe_inline"/&gt;
                          &lt;/div&gt;
                      &lt;/xpath&gt;
                  &lt;/data&gt;
              &lt;/field&gt;
          &lt;/record&gt;
      &lt;/data&gt;
  &lt;/openerp&gt;
  </code></pre>
  
  <p>This is the Python code:</p>
  
  <pre><code>from openerp.osv import osv, fields
  import openerp.addons.decimal_precision as dp
  
  # modify class sale.config.settings to insert a new field
  class my_custom_sale_config_settings(osv.osv_memory):
      _name = 'sale.config.settings'
      _inherit = "sale.config.settings"
  
      _columns = {
          'threshold_price_mgr_sig': fields.float('Threshold Price for Managers Signature', 
              digits_compute=dp.get_precision('Product Price'), 
              default_model='sale.config.settings',
              help="""Threshold price from which the managers signature is required. 
                  This is valid for quotations and order confirmations."""),
          }
  
      _defaults = {
          'threshold_price_mgr_sig': 0.00,
          }
  
      def get_default_threshold_price_mgr_sig(self, cr, uid, fields, context=None):
          last_ids = self.pool.get('sale.config.settings').search(cr, uid, [], order='id desc', limit=1)
          result_obj = self.pool.get('sale.config.settings').browse(cr,uid,last_ids[0])
          t_p_m_s = result_obj.threshold_price_mgr_sig
  
      return {'threshold_price_mgr_sig': t_p_m_s}
  </code></pre>
  
  <p>By debugging I also found, that the function <code>get_default_threshold_price_mgr_sig</code> is never executed...</p>
  
  <p>I know that configuration settings fields works different than normal fields - just can't figure out how. </p>
  
  <p>Do you know, what I am doing wrong?</p>