title: ExtJs4+: can calculated model field be editable?
body: <p>My model has two data fields: persistent and calculated:</p>
  
  <pre>
  Ext.define('My.model.Value', {
      extend: 'Ext.data.Model',
      idProperty: 'id',
      fields: [
          { name: 'id', type: 'auto' },
          { name: 'percentage', type: 'float' }, // values in 0..1 range
          { name: 'percentageDecimal', type: 'float', persist: false,
              calculate: function(v) {
                  v = v.percentage;
                  return Ext.isNumber(v) ? v * 100.0 : v;
              }
          }
      ]
  });
  </pre>
  
  <p>The <code>percentageDecimal</code> field is used in a grid and it is editable.</p>
  
  <p>Question: how can I change my model to automatically update <code>percentage</code> field everytime the <code>percentageDecimal</code> is updated?</p>
  
  <p>I use ExtJs 5.0.1.</p>