title: How to make Ext.form.Combobox cannot input "dirty" data
body: <p>Do we have any way to make an Ext.form.Combobox cannot input the data which is not in combobox's store?
  It means that user can type to field for filter items quickly, but they cannot input the "dirty" data outside of the combobox's store? Does ExtJS support this feature by configuring properties or we have to customize it?!
  Thank you very much!</p>