title: ExtJS 5: bind to other class properties, like allowBlank
body: <p>I'm getting confused on what config values are allowed to be bound, and which ones are not. Currently, allowBlank is not allowed to be bound, as I get an error saying <code>Ext.mixin.Bindable.applyBind(): Cannot bind allowBlank on Ext.form.field.ComboBox - missing a setAllowBlank method.</code> If I use a config, I don't get the error, but it doesn't work as expected:</p>
  
  <pre><code>xtype: 'combobox',
  anchor: '100%',
  fieldLabel: 'Affiliation',
  name: 'AffiliationId',
  displayField: 'Abbreviation',
  valueField: 'AffiliationId',
  queryMode: 'local',
  typeAhead: true,
  config: {
    forceSelection: true,
    allowBlank: false
  },
  bind: {
    store: '{affiliationStore}',
    allowBlank: '{allowBlankAffiliation}',
    forceSelection: '{forceSelectionAffiliation}'
  }
  </code></pre>
  
  <p>What I'd like to know is, how can I bind <code>allowBlank</code> or any other property to a formula? Is this even possible, or am I totally not understanding something?  And if so, how do I know what I can bind and what I can't bind?</p>