Filterable: Add new `filter` event

[jzaefferer]

Triggered after the hidden class is applied to items. Passes an items property on the ui argument, which is a jQuery object with all the filterable items. This can be used to apply custom classes, e.g. for zebra striping

The original ticket has sample code, look for usage of postfilter: jquery-archive/jquery-mobile#6580

See also jquery-archive/jquery-mobile#6937