How to define a custom Filter Popup allowing to add custom toolbar buttons

To accomplish this task, define a custom PivotGridField.HeaderTemplate. In the attached solution, this template is defined in the MainWindow.xaml file. The custom functionality is implemented in the ComboBoxEdit descendant. This descendant is defined in the CustomFilterPopup.cs file. Filter popup is populated from the PivotGridRadioFilter_PopupOpening event handler. To apply filter, the PivotGridRadioFilter_PopupClosed event handler is used. 
To demonstrate how to add custom functionality, the Invert command is defined.