A list model presenting filter options.
This subclasses from DataModel, and has a single column containing "Select All" and the values in source. "Select All" is tristate, indicating (and setting) the check state of all other values.
If titles is omitted, a single column is assumed.
This would commonly be used as the model of a combo or list widget to allow the user to filter out values from a data source. By default, all items are selected and any new ones added are automatically selected.
When the filter changes, a filterChanged signal is emitted with a set of unselected values.