Exclude columns from filter, make compatible with multi-column sorting

Some columns (e.g. row numbers, columns formatted to display links) should not be able to be filtered.
To ignore the filter, add unfiltered:true to the column definition:
````json
{ id: "rn", name: "row", field: "rn", unfiltered:true }
````
To remove sorting from the plugin, construct it with new option sortAvailable set to false.
````javascript
var filterPlugin = new Ext.Plugins.HeaderFilter({sortAvailable: false});
````
A third example page shows off these features as well as using search and filtering together.