Add API to check if backend was used previously

This is backported from LiveTrax, yet potentially handy
for future auto-connect policy in Ardour as well.

libs/ardour/ardour/session.h

@@ -261,6 +261,7 @@ public:
 	bool cannot_save () const                      { return _state_of_the_state & CannotSave; }
 	bool in_cleanup () const                       { return _state_of_the_state & InCleanup; }
 	bool inital_connect_or_deletion_in_progress () const { return _state_of_the_state & (InitialConnecting | Deletion); }
+	bool have_external_connections_for_current_backend (bool tracks_only = true) const;
 	bool unnamed() const;
 
 	void end_unnamed_status () const;

libs/ardour/session.cc

@@ -8158,3 +8158,27 @@ Session::foreach_route (void (Route::*method)())
 	}
 }
 
+bool
+Session::have_external_connections_for_current_backend (bool tracks_only) const
+{
+	std::shared_ptr<RouteList const> rl = routes.reader();
+	for (auto const& r : *rl) {
+		if (tracks_only && !std::dynamic_pointer_cast<Track> (r)) {
+			continue;
+		}
+		if (r->is_singleton ()) {
+			continue;
+		}
+		for (auto const& p : *r->input()->ports()) {
+			if (p->has_ext_connection ()) {
+				return true;
+			}
+		}
+		for (auto const& p : *r->output()->ports()) {
+			if (p->has_ext_connection ()) {
+				return true;
+			}
+		}
+	}
+	return false;
+}