When deriving from the GroupHeaderControl class, PrepareDefaultStyleKey must be overridden in order to support dynamic style (view or theme modifications) changes.
This call is used only when the datagrid's group selection feature is enabled via the IsGroupSelectionEnabled property.
Since selection is often handled by the datagrid's low level hardware events, this method is called to identify if the event's target element should prevent any selection handling. (e.g. a MouseDown on a button displayed inside the GroupHeaderControl)
Be default, this call returns true if the target child is, or is within, an instance of the "System.Windows.Controls.Control" class.
A common scenario for overriding this method is when a subclass of Control was added in the GroupHeaderTemplate, and interaction with that child (e.g. a mouse click) should not prevent group selection to occur.
