Suppress focus loss on opening combobox popup in wxGTK

Make GTKHandleFocusOut() virtual and override it in wxChoice in order to
avoid generating wxEVT_KILL_FOCUS events when the combobox dropdown
button is clicked.

This is important because it allows fatal problems when using a
combobox-based in-place editor in wxDataViewCtrl as getting these events
totally broke the UI before.

See #17034.

docs/changes.txt

@@ -206,6 +206,7 @@ wxGTK:
 - Implement XYToPosition() for single-line wxTextCtrl.
 - Implement ShowPosition() for single-line wxTextCtrl.
 - Improve wx{Client,Paint,Screen,Window}DC::GetPPI() (GTK+ 3).
+- Suppress focus loss events for wxChoice and wxComboBox on opening popup.
 
 wxMSW: