Previously, wxEVT_{SET,KILL}_FOCUS event handlers were connected to all
children, even the indirect ones, which resulted in duplicate focus
events if a grandchildren forward its focus event to its parent window,
as happens e.g. with wxComboCtrl, which contains a text control that
forwards its focus events to the containing control itself.
This change seems globally sound, even if it might break some controls
whose children don't get the expected focus events themselves, because
the right thing to do is to fix those children. And it fixes duplicate
focus events in wxDatePickerCtrlGeneric.
06d2b836a0