If the AllowFilter property is set to true the gallery group filtering feature is enabled. In this mode, when a dropdown gallery contains two or more groups the Group Filter panel is displayed at the top of the dropdown gallery. When this panel is clicked, a Group Filter menu is opened, which by default, contains check boxes that correspond to the existing gallery groups and which control the visibility of the groups. Unchecking a check box hides a corresponding group; checking the check box makes the group visible.
The caption of the Group Filter panel is specified by the FilterCaption property.
