Simplify GTK+ 2 version checks with a helper function

Use wx_is_at_least_gtk2(minor) instead of gtk_check_version(2, minor)
because it is more clear and also works as expected (i.e. returns true)
when using GTK+ 3 instead of creating difficult to notice bugs in the
code by failing the version check in this case.

See #18004.

src/gtk/combobox.cpp

@@ -194,9 +194,7 @@ bool wxComboBox::Create( wxWindow *parent, wxWindowID id, const wxString& value,
     g_signal_connect_after (m_widget, "changed",
                         G_CALLBACK (gtkcombobox_changed_callback), this);
 
-#ifndef __WXGTK3__
-    if ( !gtk_check_version(2,10,0) )
-#endif
+    if ( wx_is_at_least_gtk2(10) )
     {
         g_signal_connect (m_widget, "notify::popup-shown",
                           G_CALLBACK (gtkcombobox_popupshown_callback), this);