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.

include/wx/gtk/window.h

@@ -197,7 +197,7 @@ public:
     GdkWindow* GTKGetDrawingWindow() const;
 
     bool GTKHandleFocusIn();
-    bool GTKHandleFocusOut();
+    virtual bool GTKHandleFocusOut();
     void GTKHandleFocusOutNoDeferring();
     void GTKHandleDeferredFocusOut();