Fix route ordering (mixer TV lacks master-bus)

Fix route ordering (mixer TV lacks master-bus)

Skip master bus' order-key when re-ordering routes in the mixer.

This also fixes a related issue:
When a new track is added, Session::RouteAdded will call be handled by
Editor::add_routes AND Mixer_UI::add_routes, leading to calls to both:

- EditorRoutes::sync_presentation_info_from_treeview()
- Mixer_UI::sync_presentation_info_from_treeview ()

which potentially set different order-keys.