macOS provides z-axis stacking for Dialogs

see also d1b462c213 and Manager::set_transient_for.

This fixes a bug on macOS: detach Editor, re-attach Editor.
After that `own_window()` is true. Showing the LAN dialog
showed the editor's hidden Window as its parent.

gtk2_ardour/editor_export_audio.cc

@@ -184,10 +184,11 @@ Editor::measure_master_loudness (samplepos_t start, samplepos_t end, bool is_ran
 	ARDOUR::TimelineRange ar (timepos_t (start), timepos_t (end), 0);
 
 	LoudnessDialog ld (_session, ar, is_range_selection);
-
+#ifndef __APPLE__
 	if (own_window ()) {
 		ld.set_transient_for (*own_window ());
 	}
+#endif
 
 	ld.run ();
 }