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

[jcsjcs]

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:

{ id: "rn", name: "row", field: "rn", unfiltered:true }

To remove sorting from the plugin, construct it with new option sortAvailable set to false.

var filterPlugin = new Ext.Plugins.HeaderFilter({sortAvailable: false});

A third example page shows off these features as well as using search and filtering together.

•  General Comment
• <img border=0 src='https://avatars.githubusercontent.com/u/99515?v=3' height=16 width=16'> Note this also includes the same fix as for issue Fix bug with clear button on filter popup menu #3

  

[jcsjcs]

Note this also includes the same fix as for issue #3