Improve our estimate of GtkEntry margins

For GTK2, get the "inner border" the same way GTK does it. And for GTK3,
provide an actual implementation. Also, don't return a wxPoint for a size.
See #18567

include/wx/gtk/control.h

@@ -84,7 +84,7 @@ protected:
     wxSize GTKGetPreferredSize(GtkWidget* widget) const;
 
     // Inner margins in a GtkEntry
-    wxPoint GTKGetEntryMargins(GtkEntry* entry) const;
+    wxSize GTKGetEntryMargins(GtkEntry* entry) const;
 
 private:
     wxDECLARE_DYNAMIC_CLASS(wxControl);